BEIRUT — Uncertainty prevailed on Friday over the possibility of delivering badly needed aid in Syria, as a truce brokered by the United States and Russia that went into effect in the country this week was marred by further breaches.

Government forces and rebels were engaged in fierce fighting near the capital Damascus, the Syrian Observatory for Human Rights reported.

Three civilians were killed in strikes by unidentified warplanes in rebel-held Idlib province in north-western Syria, the monitoring group added.

Syria’s state news agency SANA said a church in a regime-controlled district in Aleppo was damaged after rebels fired a rocket in a breach of the truce that came into effect on Monday.

Russia, a key ally of Syrian President Bashar al-Assad, said it is prepared to extend the cease-fire, originally set for a week, for three more days.

But it added that to do so, the US needs to ensure that rebel groups it supports do not violate the truce, Russian General Viktor Poznikhir told Russian state news agency TASS.

The United States said it wanted to see definitive progress before the next stages of the cease-fire take effect.

State Department spokesman John Kirby said what was agreed last week in Geneva, including a Joint Implementation Centre, “won’t happen” unless Russia exercises its influence over Syria.

Russia has “decisions to make about the influence that they have on Assad and the degree to which they’re going to use it or not,” Kirby said.

If they apply their influence and it has no effect, “then the arrangement doesn’t come into being and we’re back to, regrettably, where we have so long been, which is innocent civilians being barrel bombed and gassed,” he said.

Earlier on Friday, rebels and government forces engaged in heavy shelling across front lines on the outskirts of Damascus, killing at least two fighters.

Nearly two dozen rockets and grenades were fired near Jobar neighborhood on the eastern outskirts of Damascus, the observatory said, adding that two shells fell in the Old City, controlled by the government.

Battles were taking place on at least two fronts, with regime forces attacking rebel positions, the watchdog said.

It was unclear who first violated the cease-fire, which has been largely holding up despite breaches.