David Cameron has said he will raise allegations of human rights abuses with Kazakhstan's president as he arrives in the country for an historic visit.
He will hold talks with President Nursultan Nazarbayev on trade and plans to use the country as an exit route for UK troops leaving Afghanistan.
But he said he would address claims of torture, the imprisonment of government critics and limits on media freedom.
Mr Cameron is the first serving British prime minister to visit Kazakhstan.
But former PM Tony Blair has been working with the Kazakh government on political, judicial and economic reform since leaving office.
Mr Cameron is in the Kazakhstani city of Atyrau on the third leg of a trip which has already included Afghanistan and Pakistan.
He has been joined by representatives of more than 30 British businesses and said he hoped to sign business deals worth more than £700m while in Kazakhstan.
But while stressing the trip was predominantly about economic matters, he said: "Nothing is off the agenda, including human rights, and Britain always stands up for human rights wherever we are in the world."
Kazakhstan has been governed by Mr Nazarbayev since the Soviet era and has recently seen a big increase in foreign investment due to its vast oil and mineral reserves.
However, the president has been labelled a dictator by critics, and in an open letter to Mr Cameron, campaign group Human Rights Watch UK said the group had been documenting human rights abuses in Kazakhstan for more than 15 years.
"We are very concerned about the serious and deteriorating human rights situation there in recent years, including credible allegations of torture, the imprisonment of government critics, tight controls over the media and freedom of expression and association, limits on religious freedom, and continuing violations of workers' rights," it said.
Amnesty International UK's head of policy and government affairs, Allan Hogarth, also said: "Kazakhstan might be knee-deep in oil and gas wealth, but David Cameron shouldn't let lucrative energy deals prevent him from raising human rights during his trip."
Arriving on Sunday, Mr Cameron said he did not accept the suggestion that he might be putting trade before human rights.
"Kazakhstan is one of the rising economic powers in the world. I think it's very important that British business, British investment and British firms get a proper chance in Kazakhstan - they're doing that, I want to help them to do that.
"Other European leaders have been and I think it's high time a British prime minister went."
Kazakhstan's foreign minister Erlan Idrissov said his country was "very honoured and privileged to have such attention on the part of two prime ministers...Tony Blair and David Cameron".
On the issue of human rights, he said: "We are a young nation so we are making our first steps. We do hear criticisms.
"We do not feel absolutely unhappy about those criticisms. We patiently explain to our partners that we are not today a Jeffersonian democracy and that a Jeffersonian democracy is our ultimate destination."
A spokesman for Mr Blair said he worked with the government of Kazakhstan "on key areas of social, political and economic reform including rule of law".
"This work is entirely in line with the work of other international organisations (for example OECD and the EU) and Western governments and follows the direction which the international community wants Kazakhstan to take."
The spokesman added that a two-year contract "funds a team of high calibre experts in London and Kazakhstan and Tony Blair does not take a personal profit from this".
Earlier in his trip, Mr Cameron promised to "stand together" with Pakistan in the fight against terrorism, after holding talks with the newly re-elected prime minister, Nawaz Sharif.
This came after Mr Cameron visited Afghanistan, where he met UK troops at Camp Bastion and held discussions with Afghan President Hamid Karzai.