A a man carries a child after airstrikes hit Aleppo last week. Fighting raged as Washington and Moscow urged calm. (AP)

The Syrian military on Monday extended a fragile cease-fire that had broken down in the northern city of Aleppo, as the United States and Russia worked together to try to get peace talks back on track and quell the violence.

Shortly before the cease-fire was due to expire and as fighting raged in the northern city of Aleppo, the Syrian military announced that the truce would be extended for 48 hours.

But Secretary of State John F. Kerry, in Paris to discuss Syrian peace talks with his counterparts from Europe and the Middle East, spoke with caution rather than optimism.

“These are words on a piece of paper. They are not actions,” Kerry said. “It is going to be up to the commanders in the field and the interested parties — which includes us.”

Earlier in the day, the United States and Russia issued a joint statement reaffirming their commitment to a truce, or cessation of hostilities, as they call it. The two countries, co-chairs of an international group pushing peace talks, pledged to use their influence with the warring parties — Russia with the Syrian government and the United States with rebel groups — to urge them to refrain from provocations and to stop firing.

“We have decided to reconfirm our commitment to the [truce] in Syria and to intensify efforts to ensure its nationwide implementation,” the statement said.

Russia said it would work with the Syrian government to minimize air operations over areas crowded with civilians or rebel groups that have agreed to the truce. That is particularly significant in areas near Aleppo, where fighters from al-Qaeda-backed Jabhat al-Nusra — which is not party to the cease-fire and is considered a legitimate target — are often based near strongholds of rebels who are parties to the cease-fire. The confusion has contributed to the unraveling of previous cease-fires.

However, Jabhat al-Nusra has only a small presence inside the city of Aleppo. Rebel groups accuse Russia of raising concerns about the al-Qaeda franchise as a pretext to attack non-extremist opponents of the government.

The cease-fire does include the thousands of Shiite militiamen from Lebanon, Iraq and Iran who are battling the mostly Sunni Muslim rebels in Aleppo on behalf of the Syrian government. The militiamen, however, have continued fighting despite the truce.

“We’d like to see a real reduction in Syrian air force overflights,” said a U.S. official, speaking on the condition of anonymity to discuss the reasoning behind the joint statement. “Even if they’re not dropping ordnance, even the hovering of a helicopter overhead, has had a worrying effect.

The statement, the official added, “makes clear neither the Russian nor Syrian air forces should strike civilians or parties to the cessation.”

But the prospects for peace talks looked dim. A 10-week-old partial truce had laid the groundwork that allowed U.N.-mediated peace talks between government and opposition representatives to begin in Geneva. But at the most recent sessions last month, both sides accused the other of violating the truce.

The government delegation to the talks appeared unwilling to compromise on key issues — a stance that diplomats familiar with the negotiations attributed to Syrian President Bashar al-Assad’s rising confidence after regaining momentum on the battlefield. His forces have received a significant boost from Russia, which has carried out devastating air raids against rebel forces.

In the joint statement Monday, the United States and Russia vowed to “redouble efforts” to reach a political settlement in the faltering talks.

They specifically mentioned the need for a political transition from the Assad government.

The mention of a transition, although not new, was considered crucial to assure rebels that they have something to gain in the end.

Morello reported from Washington, and Naylor reported from Beirut.