US President Barack Obama was on Monday set to announce plans to send up to 250 more special forces to Syria, stepping up military assistance to rebels as a ceasefire falters.
Obama was in Germany for talks with Chancellor Angela Merkel, and both were to be joined later by the leaders of Britain, France and Italy in a meeting expected to focus on the fight against the Islamic State group.
On Sunday Obama had pressed for all parties to the Syrian conflict, including the regime's ally Russia, to return to the negotiating table and "reinstate" an internationally-brokered ceasefire.
"I spoke to President Vladimir Putin early last week to try to make sure that we could reinstate the cessation of hostilities," Obama told a news conference in the northern city of Hanover.
That was the clearest indication yet that the White House believes the ceasefire has all but disintegrated as regime and rebel bombardments claimed 26 lives Sunday.
Eight weeks into the declared truce between President Bashar al-Assad's regime and non-jihadist rebels, violence has escalated around Aleppo, with dozens killed by government air strikes and rebel rockets.
The surge in fighting and stalled peace talks in Geneva have dimmed hopes that the ceasefire would lay the groundwork for finally resolving Syria's devastating five-year conflict.
The White House has argued that the ceasefire, while imperfect, is worth pursuing and is the only way out of the brutal war, which has sparked a major refugee crisis in the region and Europe.
But its stance is bringing Washington and its allies into ever more conflict with rebel groups on the ground, which continue to be on the receiving end of regime attacks.
Deputy National Security Advisor Ben Rhodes said Obama would on Monday announce his decision to deploy to Syria another 250 special forces.
"This brings the number of US special forces to roughly 300," he said.
US troops in Syria are mandated to advise and assist Syrian rebel and anti-Islamic State forces.
"What we have seen is the small team that we have put in to Syria several months ago has been very effective in serving as a force multiplier. They are able to provide advice and support to the forces that are fighting against ISIL on the ground," Rhodes said, using another acronym for IS.
Pressure on Obama is increasing in the United States, which in is the throes of a fiercely fought presidential election race, and from European allies who want to halt the massive influx of refugees.
Many of Obama's critics have called for a safe zone to be established.
But he insisted that his refusal "is not a matter of an ideological objection on my part".
"As a practical matter, sadly, it is very difficult to see how it would operate short of us essentially being willing to militarily take over a big chunk of that country."
Obama has come under criticism for his handling of Syria's war, with opponents saying he could have done more to stem the bloodshed.
But the US president -- who came to power vowing to withdraw US troops from Iraq and Afghanistan -- has stood fast in his opposition to plunging the United States into another ground war in the Muslim world.
In an interview with the BBC aired Sunday, Obama said that "it would be a mistake for the United States, or Great Britain, or a combination of Western states to send in ground troops and overthrow the Assad regime."
He urged all parties "to sit down at the table and try to broker a transition".
Syria's conflict, which began in March 2011 with widespread anti-Assad protests, has since spiralled into a multi-front war that has killed 270,000 people and drawn in regional and world powers.
As fighting has flared again, at least 27 civilian deaths were reported in regime bombardments across Syria on Saturday, and a fresh barrage of air strikes hit Aleppo on Sunday.
The Observatory said four more civilians died Sunday in strikes on other opposition neighbourhoods, and in Aleppo's western government-held parts, 10 civilians including a woman and two children were killed by rebel rocket fire.
Also Sunday, rockets fired into Turkey from an area of Syria controlled by the Islamic State group killed one person and wounded 26, Ankara's deputy prime minister said.
Turkey has killed almost 900 alleged IS members since January through artillery fire and air raids, the state-run Anatolia news agency said Monday, citing military sources.