Syria violence: US warns citizens to leave
The US has warned its citizens to leave Syria, as the government of President Bashar al-Assad intensifies its campaign against peaceful protests.
The state department also said some non-essential embassy staff and all embassy dependants would be leaving.
The UK says it is working with its partners on measures against Damascus. The US is also considering sanctions.
Tanks have been sent into Deraa, the town at the centre of protests against President Assad.
There have been numerous reports of crackdowns and arrests around Syria over recent days, despite the lifting of an emergency law last week.
More than 350 people have been killed in the violence since mid-March, activists say.
In a statement carried by the official news agency, the Syrian government said it had sent troops to several cities on the request of citizens who were worried about "armed extremists".
Arrests were made and those people would be processed through the civil courts, the government statement added, following the lifting of emergency laws last week.
The agency said some soldiers were killed in the fighting as the army moved into the southern city of Deraa.
The BBC's Owen Bennett-Jones, in neighbouring Lebanon, says communication with Syria is virtually impossible, though there are reports of continuing clashes as the government tries to re-establish control of the city.
Gunfire can be heard on video footage which has been put on the internet and is said to have been filmed on Tuesday.
On Monday witnesses said the army had advanced into Deraa, using tanks to support troops. Security forces also reportedly opened fire in a suburb of Damascus.
"The bullets continue against the people, but we are resisting," local activist Abazid Abdullah told AFP news agency.
The British Foreign Secretary, William Hague, has called for an end to the "violent repression" of protests in Syria, and said London was working to send a "strong signal" to Damascus.
"This includes working with our partners on the United Nations Security Council to send a strong signal to the Syrian authorities that the eyes of the international community are on Syria, and with our partners in the European Union and the region on possible further measures."
According to a UN Security Council diplomat, the UK and other European states are circulating a draft statement condemning the violence in Syria.
The US State Department issued a warning later on Monday urging all Americans to defer all travel to Syria and advising those in the country to leave while commercial transport was available.
The statement said government restrictions made it difficult to assess the security situation, adding that the authorities' attempts to blame violence on foreigners could endanger US citizens.
The Obama administration is considering sanctions to put pressure on the government of President Bashar al-Assad to stop its crackdown, US officials say.
The steps could include a freeze on the assets of Syrian leaders and a ban on business dealings in the US, but no time scale has been given for the measures.
Opposition activists said Monday's raid on Deraa involved as many as 5,000 soldiers and seven tanks.
Tanks surrounded the Omari mosque in the old city with snipers firing from rooftops, anonymous opposition sources said.
The opposition said more than 25 people had been killed. This claim could not be independently verified.
Deraa is the city where protesters began calling for political reforms last month. Many are now demanding that President Assad step down.