At least 5,000 people are thought to have died since the first round of Geneva talks started on 22 January. Photo: BBC NEWS

Lakhdar Brahimi is holding final discussions with both sides in Geneva.

Negotiations have reached an impasse as the two delegations traded accusations, with Syrian officials calling the rebels' demands "unrealistic".

The conflict in Syria has claimed more than 100,000 lives since March 2011.

Some 9.5 million people have been forced to flee their homes.

'Unblock the situation'

The final session began after 11:00 (10:00 GMT).

The BBC's Imogen Foulkes in Geneva says the face-to-face talks seem to have gone badly.

Sources said Mr Brahimi was unable to set a date for a third round of talks, though he expects there still to be one.

Earlier he said US and Russian officials had assured him they would try to "unblock the situation".

The best outcome the UN can hope for is a commitment to come back to Geneva for further negotiations, our correspondent adds.

Many people inside Syria may view such a commitment as meaningless after two rounds of talks achieved little, our correspondent says.

So far, the only agreement in the latest negotiations was to allow civilians to leave and aid to enter the besieged city of Homs.

Otherwise the talks, which started six days ago, have failed to narrow the gap between the government of President Bashar al-Assad and the opposition.

Syria insists on the need to fight what it calls "terrorists", while the rebel delegation stresses the need for a transitional administration to run the country until elections.

Syrian officials have said there is no question of replacing President Bashar al-Assad.

Opposition spokesman Louay Safi said the government team had failed to show "any responsiveness".

Correspondents say at least 5,000 people are believed to have died since the first round of the Geneva talks began on 22 January.

US President Barack Obama said he was considering ways of putting more pressure on President Assad, though he did not expect any resolution in the short term.

Speaking in California, where he was meeting Jordan's King Abdullah, he said: "There will be some intermediate steps that we can take applying more pressure to the Assad regime and we are going to be continuing to work with all the parties concerned to try to move forward on a diplomatic solution."

Mr Obama did not disclose what steps he has under consideration.

Earlier, UN humanitarian chief Valerie Amos urged the UN Security Council to act immediately to ensure more humanitarian access in Syria.

The Security Council has been deadlocked over aid deliveries in Syria, where millions have been forced to flee their homes.