Two Syrian rebel groups shot down a warplane in southern Syria on Tuesday, the groups announced in a joint statement. The shooting took place over territory belonging to the government of Syrian President Bashar al-Assad and came dangerously close to a cease-fire zone brokered last week by the U.S. and Russia. The ceasefire, which went into effect on Sunday, applies to the Daraa, Quneitra, and Sweida regions in southern Syria. On Tuesday, the Syrian Observatory for Human Rights, a U.K.-based war monitor, confirmed that rebels downed a government plane near a village bordering Sweida and Damascus.

“The plane was shot down and crashed in regime-controlled territory. We have no information on the pilot,” said Fares al-Munjed, the head of communications for Ahmad al-Abdo, one of the rebel groups. Both Ahmad al-Abdo and the other group responsible for the shooting, Lions of the East Army, belong to a coalition of rebel forces known as the Free Syrian Army. For more than six years, Syrian rebels—some of them backed by Turkey and the U.S.—have been embroiled in a massive civil war with Assad regime. Since the beginning of the war in 2011, hundreds of thousands of civilians have been killed and more than half of Syria’s population—around 11 million people—have been displaced.

Last week, President Trump and Russian President Vladimir Putin struck a deal to suspend the violence in southern Syria during a two-hour conversation at the G20 summit in Hamburg, Germany. While it’s unclear exactly how the nations plan to enforce the agreement, Russian Foreign Minister Sergey Lavrov said the Russian military will secure the region, while the U.S. and Russia, along with Jordan, will oversee potential violence from a joint monitoring center. Many have since questioned the efficacy of the deal, which was negotiated without the presence of rebel groups or the Syrian government.

In the past, both the Syrian government and rebel groups have been hesitant to sign on to cease-fire agreements negotiated by third parties. In December, a deal approved by Russia was violated within hours of taking effect. Months later, Russia, Iran, and Turkey established four safe zones across Syria, which were repeatedly violated on both sides. By June, the Syrian Network for Human Rights, a U.K.-based watchdog, had documented 75 violations within the first month of the agreement.

Still, U.S. officials insist that the latest cease-fire represents progress in the region. On Sunday, the same day that the cease-fire went into effect, Trump reported that the agreement “seems to be holding” and that “many lives can be saved.” A day later, the UN’s special envoy for Syrian peace talks, Staffan de Mistura, said the cease-fire was experiencing “teething problems,” but could ultimately result in “some incremental developments.” Monday saw dispersed instances of violence throughout the region, including a government-led attack on Ahmad al-Abdo and the Lions of the East Army, which was confirmed by the Syrian Observatory. On Tuesday, the head of the observatory, Rami Abdel Rahman, told Agence France-Presse that the fighting continued, but no casualties had been reported.

While small bouts of violence could represent a hiccup in an otherwise promising cease-fire deal, they may also signal the start of mounting violations similar to those recorded during past agreements. Whatever the case, the Trump administration hopes that a cease-fire will lead to greater cooperation with Russia in the region, a senior State Department official told ABC News. By establishing a peaceful infrastructure in the south, the official said, the U.S. might encourage de-escalation in other parts of Syria.