Syria conflict: Truce in danger of collapse - Obama and UN
President Obama and UN Syria envoy Staffan de Mistura have said they fear the fragile truce in Syria is collapsing.
In the latest violence, 18 people were reported to have died in government air strikes in Aleppo and a Syrian warplane was said to have crashed near Damascus.
Speaking in Geneva, Mr de Mistura said he wanted to call a regional summit to prevent peace talks failing.
He said he thought that 400,000 people may now have died in the conflict.
The UN diplomat said he had "no proof" for the figure but said he feared it was not far off.
He said the air strikes on rebel-held areas of Aleppo showed there were some "very worrisome trends on the ground" and said "urgent efforts" were needed to get the cessation of hostilities "back on track".
Speaking during a visit to the UK, Mr Obama said he was "deeply concerned" about whether the truce was "sustainable".
Call for regional talks
Talks in Geneva with the Syrian government and opposition delegations would continue into next week, Mr de Mistura said.
But he said a ministerial meeting of the International Syria Support Group (ISSG) - including the US, Russia, the EU, Iran, Turkey and Arab states - was now needed to save the truce.
"None of the sides have renounced it, delegitimised it, and it is still in effect. But it is in great trouble if we don't act quickly," he said.
The Syrian opposition delegation in Geneva, known as the High Negotiations Committee (HNC), and rebel forces inside Syria have accused the government of repeatedly violating the terms of the truce brokered by the US and Russia.
On Monday, rebels said they had begun new attacks on the army in response to alleged violations.
In Geneva the HNC said the government was "not a serious partner" while the government hit back by accusing the HNC of "sulking".
Mr Obama said he would try to keep the truce in effect and stressed that even if it collapsed he would work to put it back together.
He said that although Syrian President Bashar al-Assad's backers had succeeded in keeping him in power, they also knew they could not defeat the opposition militarily.
"We cannot end the crisis in Syria without political negotiations and without getting all the parties around the table to craft a transition plan," he said.
Meanwhile activists said fighting was continuing in several parts of Syria.
In addition to the deaths in Aleppo, the Syrian Observatory on Human Rights - a UK-based monitoring group - said seven people had died in clashes between the army and Kurdish forces in the north-eastern town of Qamishli.
A government warplane crashed near the capital Damascus on Friday.
Fighters from the Islamic State group said that they captured the pilot, named as Azzam Eid, after he parachuted from the plane, according to the IS-affiliated news agency Amaq.
IS said it had shot down the plane, but the Russian news agency Interfax, quoting Syrian military sources, said it had crashed because of a technical fault.