Of the six F-16 fighter jets that Thailand readied to deploy along the disputed border, one of the aircraft fired into Cambodia and destroyed a military target, the Thai army said. Both countries accused each other of starting the clash early on Thursday.

"We have used air power against military targets as planned," Thai army deputy spokesperson Richa Suksuwanon told reporters. Thailand also closed its border with Cambodia.

Cambodia's defense ministry said the jets dropped two bombs on a road, and that it "strongly condemns the reckless and brutal military aggression of the Kingdom of Thailand against the sovereignty and territorial integrity of Cambodia".

The skirmishes came after Thailand recalled its ambassador to Cambodia late on Wednesday and said it would expel Cambodia's envoy in Bangkok, after a second Thai soldier in the space of a week lost a limb to a landmine that Bangkok alleged had been laid recently in the disputed area.

Thailand's foreign ministry said Cambodian troops fired "heavy artillery" on a Thai military base on Thursday morning and also targeted civilian areas including a hospital, leading to civilian casualties.

"The Royal Thai Government is prepared to intensify our self-defense measures if Cambodia persists in its armed attack and violations upon Thailand's sovereignty," the ministry said in a statement.

Thai residents including children and the elderly ran to shelters built of concrete and fortified with sandbags and car tires in the Surin border province.

"How many rounds have been fired? It's countless," an unidentified woman told the Thai Public Broadcasting Service (TPBS) while hiding in the shelter as gunfire and explosions were heard intermittently in the background.

Cambodia's foreign ministry said Thailand's air strikes were "unprovoked" and called on its neighbor to withdraw its forces and "refrain from any further provocative actions that could escalate the situation".

For more than a century, Thailand and Cambodia have contested sovereignty at various undemarcated points along their 817-km land border, which has led to skirmishes over several years and at least a dozen deaths, including during a weeklong exchange of artillery in 2011.

Tensions were reignited in May following the killing of a Cambodian soldier during a brief exchange of gunfire, which escalated into a full-blown diplomatic crisis and now has triggered armed clashes.

The clashes began early on Thursday near the disputed Ta Moan Thom temple along the border between Cambodia and Thailand, around 360 km east of the Thai capital Bangkok.

Thailand's military said in a statement that nine people have been killed across three border provinces, including an 8-year-old boy in Surin.

"Artillery shell fell on people's homes," Sutthirot Charoenthanasak, district chief of Kabcheing in Surin province, told Reuters, adding that district authorities had evacuated 40,000 civilians from 86 villages near the border to safer locations. "Two people have died," he added.

Video footage showed a plume of thick black smoke rising from a gas station in the neighboring Thai Sisaket province, as firefighters rushed to extinguish the blaze.

Six people were killed and 10 wounded at the site, the military said, adding another person was killed in the border province of Ubon Ratchathani.

"The Thai Army condemns Cambodia for using weapons to attack civilians in Thailand. Thailand is ready to protect sovereignty and our people from inhumane action," it said in a statement.

The military said Cambodia deployed a surveillance drone before sending troops with heavy weapons to an area near the Ta Moan Thom temple.

Cambodian troops opened fire and two Thai soldiers were wounded, a Thai army spokesperson said, adding Cambodia had used multiple weapons, including rocket launchers.

A spokesperson for Cambodia's defense ministry, however, said there had been an unprovoked incursion by Thai troops and Cambodian forces had responded in self-defense.

Thailand's acting Prime Minister Phumtham Wechayachai said the situation was delicate.

"We have to be careful," he told reporters. "We will follow international law."

An attempt by Thai premier Paetongtarn Shinawatra to resolve the recent tensions via a call with Cambodia's influential former Prime Minister Hun Sen, the contents of which were leaked, kicked off a political storm in Thailand, leading to her suspension by a court.

Hun Sen said in a Facebook post that two Cambodian provinces had come under shelling from the Thai military.

Thailand this week accused Cambodia of placing landmines in a disputed area that injured three soldiers. Phnom Penh denied the claim and said the soldiers had veered off agreed routes and triggered a mine left behind from decades of war.

Cambodia has many landmines left over from its civil war decades ago, numbering in the millions according to de-mining groups.

But Thailand maintains landmines have been placed at the border area recently, which Cambodia has described as baseless allegations.