Cambodia and Thailand exchanged fire along their contested border on Thursday, bringing relations between the two Southeast Asian countries to the lowest point in decades.

Tensions at the border have been rising for months, contributing to a brewing political crisis in Thailand that helped lead to the suspension of the prime minister, Paetongtarn Shinawatra, this month.

Here are live updates.

Shots were heard early on Thursday morning near Prasat Ta Muen Thom, an ancient temple on the Thai side of the border. The countries have long sparred over the ownership of the temple.

Each nation accuses the other of firing first.

Ms. Paetongtarn said on social media that Cambodian forces had “initiated the use of arms and fired into Thai territory.” The Thai army said that Cambodia had fired rockets into civilian areas in four Thai provinces, and that it sent F-16 fighter jets to strike targets in Cambodia in response. Thai authorities ordered an immediate evacuation of a border area in Surin Province.

Prime Minister Hun Manet of Cambodia said that the country had “no choice but to respond with armed force against this armed invasion.”

Cambodian officials said that Thai soldiers had opened fire on Cambodian troops first at the temple, and that Cambodian troops returned fire about 15 minutes later.

Thai officials said that 11 civilians and one soldier were killed in the clashes and at least 31 people were injured, including a 5-year-old boy. Thousands of residents have begun evacuating from the area. Cambodian officials did not immediately confirm reports of casualties.

Longstanding tensions between the countries began to escalate in late May after a skirmish between Thai and Cambodian troops along the border, which led to the death of a Cambodian soldier.

On Wednesday, a Thai soldier lost his leg when a land mine exploded near the border. Thailand then said that it would downgrade diplomatic relations with Cambodia, recalling its ambassador to the country and expelling Cambodia’s ambassador.

In June, Ms. Paetongtarn called Hun Sen, Cambodia’s de facto leader and father of the current prime minister, to discuss the escalating border tensions. Mr. Hun Sen has close ties with her father, Thaksin Shinawatra, a former prime minister of Thailand who is the leader of a powerful political dynasty, as well as one of the country’s richest men.

Mr. Hun Sen posted a recording of their call, in which Ms. Paetongtarn seemed to both disparage Thailand’s powerful military and take a deferential tone. She called Mr. Hun Sen “uncle” and told him that she would “arrange” anything that he wanted.

In response, thousands of protesters took to the streets of Bangkok to express their outrage. Although Ms. Paetongtarn apologized, she has faced pressure to resign less than a year after taking office, including from within her governing coalition.

In early July, a Thai court suspended her.

The two countries have had occasional military clashes and nationalist rivalries for hundreds of years. The border disputes can be traced back to a 1907 map created during French colonial rule in Cambodia. The two countries interpret the map differently.

Military fighting has broken out intermittently since 2008, but the last time that a major clash turned deadly was in 2011.

In that year, about a week of fighting in disputed territory killed at least 15 people, including civilians, and displaced tens of thousands of civilians. The fighting focused on a jungle border area that included ancient temples, to which both sides had laid claim.

Here is a timeline of the tensions.