The UK is ready to escalate sanctions against Russia in the event of a full invasion of Ukraine, the foreign secretary has said.
Liz Truss defended existing measures, describing them as the most severe the UK had ever placed on Russia.
She said it was important to keep further sanctions "in the locker".
Russian President Vladimir Putin has ordered troops to be sent into two Ukrainian regions held by Russian-backed separatists.
On Tuesday Prime Minister Boris Johnson announced sanctions against Russia in response, with five banks having their assets frozen, along with those of three Russian billionaires who will also be hit with UK travel bans.
But a number of MPs, including some Conservatives, have called for tougher measures.
During Prime Minister's Questions, Labour leader Sir Keir Starmer pressed Mr Johnson to bring in a "full package of sanctions" immediately, saying Russia had already invaded.
"There is clearly concern across the house that his strategy, I accept unintentionally, could send the wrong message," he added.
The prime minister defended the government's actions, saying it was ready to escalate sanctions "very rapidly" but it was "vital" to work in "lockstep" with allies.
Downing Street will hold a summit later with finance chiefs and regulators to discuss how to make sanctions effective.
Mr Johnson announced the UK would be providing a further package of military support to Ukraine shortly.
"This will include lethal aid in the form of defensive weapons and non-lethal aid," he said.
He also confirmed the culture secretary had asked media regulator Ofcom to review the licence of RT - a Russian state-controlled TV network formerly known as Russia Today - to operate in the UK.
Meanwhile, Home Secretary Priti Patel said the government was working to prevent Russian money obtained through corrupt practices coming in to the UK.
She acknowledged that "for too long London has been the place that people have come to wash dirty money" and said stronger legislative tools were needed to tackle this.
Ms Truss told BBC Breakfast the sanctions were "coordinated" with the US and Europe "to make sure that Vladimir Putin cannot play divide and rule amongst the allies".
"We've shown we are united and we will escalate those sanctions in the event of a full invasion into Ukraine," she added.
Ms Truss said she believed Mr Putin was "hell-bent on invading Ukraine" and that this was a long-standing plan.
She reiterated comments made by Mr Johnson that football's Champions League final should no longer be held in the Russian city of St Petersburg.
Elsewhere, in unguarded comments made while chatting to military personnel at an event in London, Defence Secretary Ben Wallace said Mr Putin had gone "full tonto" over Ukraine and had "no friends" internationally.
Earlier, Ms Truss also defended donations made by Russians to the Conservative Party, saying there was a distinction between those who had moved to the UK years ago and those who were actively supporting Mr Putin's government.
She was shown a picture from May 2019 which showed her, former Prime Minister Theresa May and other cabinet ministers at the time with Lubov Chernukhin, the wife of a former Russian minister who has been financially linked to people who were close to the Kremlin.
Mrs Chernukhin has been one of the Conservative Party's biggest donors. Her lawyers say the donations are not tainted by Kremlin influence.
Asked if she was embarrassed by the picture, Ms Truss said "no, I'm not".
Has the UK gone far enough?
At the start of this month, Prime Minister Boris Johnson warned Russia the UK was "preparing a package of sanctions and other measures to be enacted the moment the first Russian toe cap crosses further into Ukrainian territory".
Fast forward to today - and the view in London is very much that toe-caps have crossed that threshold.
The prime minister believes an invasion is under way, even if Downing Street says it's not a full-scale one yet and is still urging Russia to stand back from the brink.
So, has the UK followed through on its threats about sanctions?
Sources have told the BBC that the foreign secretary has written up a "long list" of Russian individuals and organisations, including more oligarchs and banks close to the Kremlin, to be sanctioned if the situation in Ukraine gets worse.
The Foreign Office is also likely to target Russian companies in the defence, energy, technology and chemical sectors.
Other measures announced by the Foreign Office on Tuesday include sanctioning Russian parliamentarians and preventing British firms from doing business in the two rebel-held areas in Donetsk and Luhansk, recognised by Russia as independent.
Ms Truss also said the UK would restrict the Russian state and key companies from raising funding in UK financial markets, as well as ban a range of high-tech exports to Russia, "degrading the development of its military-industrial base for years".
In a statement released on Wednesday, Ms Truss reaffirmed the UK's commitment to guarantee up to $500m (£368m) in loans to Ukraine to mitigate the economic effects of Russian aggression.
Lord Kim Darroch, a former UK ambassador to the US, said the UK's sanctions appeared "pretty modest" compared to the EU's and particularly Germany's, but he understood the desire to "hold some things back" to deploy in the event of an escalation.
Tom Keatinge, director of RUSI's Centre for Financial Crime and Security Studies, described the UK's sanctions as "essentially taking a pea-shooter to a gun-fight".
Perhaps the biggest penalty Russia has faced so far is the decision by German Chancellor Olaf Scholz to halt approval of the Nord Stream 2 gas pipeline between the two countries.
The US and EU have both introduced a range of sanctions, including restricting the Russian state from financing its debt from their banks and banning their own companies from trading with the two rebel-held regions.
They have also targeted key Russian individuals and organisations with sanctions.
Who and what are being targeted by UK sanctions?
Putin ally Gennady Timchenko is a billionaire hockey enthusiast with wide business interests in energy, transportation and construction via his Volga investment group.
The UK government has accused him of supporting a Russian policy of destabilising Ukraine through his association with Bank Rossiya, of which he is a major shareholder.
Igor Rotenberg, another close associate of President Putin, is the oldest son of Arkady Rotenberg and the majority shareholder in the Gazprom Drilling company.
Igor's uncle, Boris Rotenberg, has also been targeted. Mr Rotenberg is the co-owner - along with his brother Arkady - of the SGM group, the largest construction company for gas pipelines and electrical power supply lines in Russia.
The UK's sanctions notice says he is "benefiting from or supporting" the Russian state in his role in a key strategic sector.
Rossiya is a privately-owned bank, which also has stakes in a television company.
The Black Sea Bank is a Crimean bank and was created in 2014 after Russia annexed the region.
The General Bank is a financial institution which also operates in Crimea.
Promsvyazbank is a Russian state-owned bank, which the UK government says provides funding to Russia's military.
The BBC's business correspondent Theo Leggett says the UK was not targeting the biggest banks but banks known to be used by President Putin's inner circle.
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