NEW YORK -- The United States, Russia and more than a dozen other countries declared Tuesday that Syria's ceasefire wasn't dead despite increasing violence on the ground. With few alternatives for trying to end its civil war, diplomats pressed on with a strategy that appeared to be impressing few and convincing no one.

"It's the only show in town," British Foreign Secretary Boris Johnson said bluntly of the talks that are now set to continue later this week.

Tuesday's gathering, led by Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov, aimed to hold on to what might be salvageable from Syria's week-old truce on a day the pair once envisioned as the start of a new military partnership against the Islamic State group and al Qaeda.

While Arab, European and other officials waited, Kerry and Lavrov met in a New York hotel to iron out some of their many differences, including a mistaken weekend air raid by the American-led coalition on Syrian soldiers and a deadly attack on an aid convoy that the U.S. blamed on Russia and Syria, despite denials from Moscow and Damascus. When the diplomats emerged from their meeting about an hour later, no one spoke of a breakthrough.

"The ceasefire is not dead," Kerry insisted. "We are going to continue to work. We are going to meet again Friday on some specific steps."

Rhetorically, at least, the diplomats expressed some hope. That was an improvement from Monday, when Syria's Russian-backed government declared the ceasefire over.

But it also reflected Washington's desperation. Despite numerous violations by the government and Syrian rebels, the U.S. has few other options for ending the five-year-old conflict that has killed a half-million people, contributed to Europe's worst refugee crisis since the Second World War and allowed the Islamic State group to emerge as a global threat.

President Barack Obama has made clear the U.S. will use military force only against IS and other extremist organizations, and not against Syrian President Bashar Assad's government.

Kerry and Lavrov have spent months trying to forge a diplomatic path. Their deal earlier this month would have created a new, joint U.S.-Russian centre to co-ordinate strikes on the Islamic State militants and al Qaeda-linked groups had the truce and unfettered aid deliveries in Syria been protected for seven straight days. Neither commitment was met.

The International Syria Support Group the two convened includes countries with widely divergent interests, such as Iran -- a Syrian ally -- and Saudi Arabia, a provider of arms and cash to rebels. For the same reason, the assembly of nations has been unable to define any clear approach toward a Syrian future without Assad. Most of its pronouncements have been vague. Tuesday's meeting wasn't followed by any immediate statement.

UN peace envoy Staffan de Mistura said all participants reconfirmed support for the truce, even if Assad's military and the rebels weren't always respecting it. "The ceasefire is in danger, is being seriously affected," he said, but only the U.S. and Russia could declare it over.

De Mistura also said he was "profoundly outraged" by Monday's attack on an aid convoy that killed some 20 civilians, describing it as a "game changer" in forcing a serious discussion on how to stop the violence. Both Russia and Syria have denied involvement in the strike. The United Nations has suspended all aid deliveries pending a security review.

On Monday, the U.S. was even stronger in its criticism. State Department spokesman John Kirby said after the convoy attack that the U.S. "will reassess the future prospects for co-operation with Russia."

But in a sign of the increasing messiness of Syria's overlapping wars, Washington was still on the defensive itself Tuesday.

The coalition's weekend attack killed 62 Syrian soldiers. Russia and Syria have called it proof of U.S. support for extremist groups. The American military said it may have unintentionally hit the group while carrying out operations against IS; the strike was halted when Russia informed the U.S. of the apparent mistake.

"The atmosphere was quite heavy," French Foreign Minister Jean-Marc Ayrault said of Tuesday's meeting in the Palace Hotel.

Ayrault, who has criticized Washington for not releasing the ceasefire agreement, said the U.S.-Russian negotiations "have reached their limits."

"What have we seen these last few hours?" he asked. "Bombing is continuing. Aleppo is still threatened. The population is starving. And there is a humanitarian convoy that is attacked and there are dead. This is the reality. One must denounce this realty."

Associated Press writers Matthew Lee and Maria Danilova contributed to this report