Thailand launched airstrikes along the disputed border with Cambodia on Monday as both sides accused the other of breaking a ceasefire that halted fighting earlier this year.  Longstanding border disputes erupted into five days of combat in July that killed dozens of soldiers and civilians. US President Donald Trump pushed the Southeast Asian neighbours to sign a truce agreement in October, but tensions have continued to simmer.

The Thai army said that more than 50,000 people have left areas near the border for shelters, while Cambodia’s Information Minister Neth Pheaktra said that residents of several villages near the border have been evacuated. The latest round of clashes has killed at least one Thai soldier and four Cambodian civilians, officials said.

Thai Prime Minister Anutin Charnvirakul said in a televised speech that military operations would be carried out as necessary to defend the country and protect public safety. “Thailand has never wished for violence. I’d like to reiterate that Thailand has never initiated a fight or an invasion, but will never tolerate a violation of its sovereignty,” he said. Fighting followed the exchange of fire on Sunday

The ceasefire was strained in early November after Thai troops were injured by land mines, leading Thailand to announce that it would indefinitely suspend implementation of the agreement. Both sides continue to trade accusations over responsibility, even as they are supposed to be cooperating in getting rid of the mines. Trump said in mid-November that he’d intervened to preserve the ceasefire as tensions simmered between the two countries.

But another brief episode of fighting took place along the border Sunday, after which both sides said the other fired first. The Thai army said Cambodian fire injured two Thai soldiers, and Thai troops retaliated, resulting in an exchange of fire that lasted around 20 minutes. Cambodia said that the Thai side fired first and that its own troops did not retaliate.

On Monday, Thai army spokesperson Maj. Gen. Winthai Suvaree said the Cambodian troops fired first into Thai territory in multiple areas. He said at least one Thai soldier was killed and about eight other soldiers were wounded, and civilians were being evacuated from the affected areas. Thailand used aircraft “to strike military targets in several areas to suppress Cambodian supporting fire attacks,” he said.

The Thai army said artillery rounds landed near residential areas on the Thai side, but reported no civilian casualties so far. Cambodian Defence Ministry spokesperson Maly Socheata said the Thai military attacked the Cambodian troops first on Monday, and that Cambodia did not retaliate during the initial attacks. “Cambodia urges that Thailand immediately stop all hostile activities that threaten peace and stability in the region,” she said. Neth Pheaktra, the Cambodian Minister of Information, said fire from Thai forces killed four Cambodian civilians and injured about nine others.

The prime minister of regional neighbour Malaysia, Anwar Ibrahim, called for restraint in a statement posted to social media and said that his country is ready to support efforts to avert further fighting. “Our region cannot afford to see long-standing disputes slip into cycles of confrontation,” he wrote. Centuries of rivalry lie beneath the territorial dispute Thailand and Cambodia have a history of enmity going back centuries, when they were warring empires.

Their modern territorial claims stem largely from a 1907 map drawn when Cambodia was under French colonial rule, which Thailand has argued is inaccurate. The International Court of Justice in 1962 awarded sovereignty to Cambodia over an area that included the 1,000-year-old Preah Vihear temple, which still rankles many Thais.

The ceasefire does not spell out a path to resolve the underlying basis of the dispute, the longstanding differences over where the border should run. A wounded Thai soldier is carried to be transferred to a hospital in Sisaket province Thailand on Sunday after, according to a Thai army spokesperson, Cambodian troops fired into Thai territory.