Gunfire, rockets and bombs dropped by F-16 fighter jets thundered across the long-contested border between Thailand and Cambodia on Thursday, killing at least a dozen people, in a drastic escalation of hostilities between the two neighbors.

The Cambodian and Thai militaries exchanged gunfire in at least six areas in Thailand, according to Thai officials. At least 11 civilians and a soldier were killed and two dozen more people were wounded. Thousands were told to leave their homes near the border.

Thailand responded by sending F-16 jets to strike targets in Cambodia. The Cambodian authorities did not immediately release details about casualties. A group of monks there said they had abandoned their pagoda near the border.

It was the latest deadly conflict to erupt over a decades-old flashpoint between the two countries — one that is rooted in borders drawn by France when it was a colonial power. Thailand and Cambodia each claim ownership of Prasat Ta Muen Thom, an ancient temple perched on the Thai side of the forested Dangrek mountains that divide the two countries.

The event that prompted the current violence appears to have been a land mine explosion near the temple on Wednesday, in which a Thai soldier lost a leg. Thailand accused Cambodia of laying mines in Thai territory, while Cambodia accused Thai troops of venturing into its territory. The episode followed months of simmering tensions and a fatal skirmish in May that killed a Cambodian soldier.

The current dispute has already prompted a crisis in Thai politics, leading to the suspension of the country’s prime minister earlier this month.

Cambodian soldiers loading a rocket launcher in Preah Vihear Province, Cambodia, on Thursday.Credit…Agence France-Presse — Getty Images

Diplomatic relations between the two countries unraveled rapidly on Wednesday. Both sides recalled their ambassadors, and many border crossings between the countries were shut.

Phaiboon Yerngram, a rice and sugar cane farmer in a rural section of Surin Province in northeast Thailand, said she had fled with 10 relatives after hearing multiple rounds of gunfire on Thursday morning. Her husband stayed behind to take care of their cattle, she said, but was rocked by an explosion about a quarter of a mile from their home.

“In the past, they had asked us to prepare” to evacuate, Ms. Phaiboon said of the Thai authorities. “But I never believed that today’s event would happen.” She was sitting on a straw mat in an evacuation center at a university while her 9-month-old granddaughter was sleeping in a swinging crib that was zipped shut with a mosquito net.

Both sides traded accusations about who fired first on Thursday.

Cambodian officials said that Thai troops had opened fire at the temple in Surin. Prime Minister Hun Manet of Cambodia said in a statement that his nation had “no choice but to respond with armed force against this armed invasion.” He also called on the United Nations Security Council to intervene.

People flee their homes near the Cambodia-Thailand border in Cambodia’s Oddar Meanchey province on Thursday.Credit…Tang Chhin Sothy/Agence France-Presse — Getty Images

Thailand insisted Cambodia started the attack. Prime Minister Paetongtarn Shinawatra, the leader of the governing coalition, said on social media that Cambodian forces had “initiated the use of arms and fired into Thai territory — impacting both officials and innocent civilians.”

Both sides also accused the other of targeting civilian areas. People in Samrong, a Cambodian district near the border, said they were still hearing gunfire hours after the clashes began. But Thailand’s acting prime minister, Phumtham Wechayachai, said the fighting did not appear to be spreading to other regions.

Ms. Paetongtarn has tried to tamp down the tensions. In June, she spoke with Hun Sen, the de facto leader of Cambodia and the father of the current prime minister. But Mr. Hun Sen leaked the recording of their chat, in which she referred to him deferentially as “uncle” and said the powerful Thai military was on the “opposite side.”

That caused an uproar in Thailand that led to her suspension on ethics charges.

Mr. Hun Sen, analysts said, was exploiting the ongoing turmoil in Thai politics, which has pitted the Shinawatra dynasty, of which Ms. Paetongtarn is the scion, against the royalist-military establishment.

“This weakness in Thai politics allows Cambodia to provoke or incite any actions easier,” said Pongphisoot Busbarat, a security expert at Thailand’s Chulalongkorn University

On Thursday, the Thai Army said that a Cambodian rocket had struck a gas station convenience store in Sisaket Province, killing six people and injuring 10 others. Cambodian news media reported that the Thai air raids had hit the Preah Vihear Temple, a UNESCO World Heritage Site on the Cambodian side of the border.

The sharp escalation in the conflict brought fears of a broader fallout in the region, and there were widespread calls for both sides to show restraint. The violence was being closely watched by neighboring countries like Vietnam, as well as the rest of the Association of Southeast Asian Nations, known as ASEAN, a 10-member regional bloc that counts Thailand and Cambodia as members.

The Phanom Dong Rak Hospital in Surin Province was damaged on Thursday.Credit…Sunny Chittawil/Associated Press

Anwar Ibrahim, the prime minister of Malaysia and the current chair of ASEAN, said he had spoken by phone with the leaders of Thailand and Cambodia on Thursday. In a Facebook post, he said that both had appeared willing to hold talks and that he had appealed for an immediate cease-fire.

“I welcome the positive signals and willingness shown by both Bangkok and Phnom Penh to consider this path forward,” he said.

An official in Beijing said that China was trying to facilitate talks. The fighting could present a diplomatic opening for China, which has increased its economic and political influence in both countries at a time when governments in Southeast Asia are becoming increasingly wary of the United States.

The last time clashes between Thailand and Cambodia turned deadly over the Prasat Ta Muen Thom temple was in 2011. At the time, a week of fighting in disputed territory killed at least 15 people, including civilians, and displaced tens of thousands of civilians.

Ms. Phaiboon, 58, who ended up with her granddaughter and other relatives at the evacuation center at Surindra Rajabhat University, lamented the deterioration of ties between Thailand and Cambodia.

“The Thai people always traveled back and forth,” she said. Speaking of the Cambodians, she added: “They could come and visit us, we could go and visit them. I don’t know whether it’ll ever be safe again.”

Phuriphat Dejsuphong contributed reporting from Surin Province, Thailand; Sun Narin from Oddar Meanchey Province, Cambodia; and Kittiphum Sringammuang from Bangkok.