The tiny Central American country of Belize has accused its larger neighbour Guatemala of “amassing” troops along the two countries’ border following the death of a 13-year-old boy in a shooting incident apparently involving Belizean soldiers.

Guatemala responded on Friday with a scathing statement lamenting the Belizean military’s “aggressive attitude”, saying its “acts of violence” were hurting bilateral relations.

The shooting has quickly turned into a diplomatic tussle between the two countries and the latest flashpoint in a longstanding territorial dispute. Guatemala claims parts of territory governed by Belize as its own.

Guatemala says teenager Julio René Alvarado Ruano, his father and his 11-year-old brother were attacked on Wednesday as they planted crops in the border community of San José Las Flores near Melchor de Mencos, which is on the border between the department of Petén and Belize.

The Guatemalan president, Jimmy Morales, called it a “cowardly and excessive attack” that merits the “total condemnation of the Guatemalan state”. He said he had summoned his ambassador to Belize for consultations and urged Belize to investigate and bring those responsible to justice.

Belize’s government said in a statement that according to initial reports, its security forces were investigating illegal land clearing in the Cebada area of the Chiquibul national park in western Belize when they detained a Guatemalan man suspected of illicit activities.

It said the patrol came under fire around nightfall and shot back in self-defence. Before leaving the location just inside Belizean territory, the soldiers found the boy’s body, which was taken to Belize City for an autopsy, the statement said.

The detained man, identified as José María Antonio Reyes, was handed over to police and was awaiting arraignment on unspecified charges.

The Belizean prime minister, Dean Barrow, disputed Morales’s version of events, saying in a statement that his country “has a long history and tradition as a peace-loving country, respectful of international law [and] human rights.”

Belize said its soldiers had been accompanied by members of a Belizean conservation group when the incident occurred.

The Belize government said in a press statement late on Thursday that “the current amassing of Guatemalan troops in the border areas … only adds volatility to the tensions.”

Morales did not specifically mention troop mobilisations, but said: “We have decided that from this moment on, we will carry out a strict exercise of protection” of the border area.

On Friday, the Guatemalan foreign ministry said the autopsy conducted in Belize determined that the boy had been shot eight times, including four times in the back, by a high-powered, military-grade rifle. It also cited nine other Guatemalans it said had been killed by Belizean forces since 1999.

“It is regrettable that the defence forces of Belize are the only army in the Latin America and Caribbean region that fires on unarmed civilians from another country,” the ministry said in a statement.

In Washington, the Organization of American States announced it will investigate the death at the request of both governments.

The OAS urged both sides to avoid “an escalation of tensions” and said they should “work even harder to establish a lasting peace”.

Guatemala and Belize, a former British colony, have a territorial dispute that dates back more than 150 years. Guatemala recognised Belizean independence in 1991 but still claims parts of the country’s territory as its own.