An Indian fighter jet shot down a Pakistani military plane this morning near the Indo-Pakistani border, killing all 16 people on board and setting off a barrage of recriminations between the rival countries less than three weeks after they ended their most serious border conflict in three decades.

Pakistani authorities asserted that the plane, a French-made Dassault-Breguet Atlantique naval warfare and sea reconnaissance aircraft, was unarmed and conducting a "routine training mission." They said it had not crossed into Indian airspace when it was shot down just around 11 a.m. local time (2 a.m. EDT) near Pakistan's southern province of Sindh and the Indian state of Gujarat.

The Pakistani high commissioner here, the equivalent of an ambassador, called the incident "a wanton and cowardly act" of "cold-blooded murder," and said Pakistan "reserves the right to make an appropriate response in self-defense." But Pakistani Prime Minister Nawaz Sharif made no immediate comment other than to express regret for the loss of life, and some analysts said Pakistan was seeking to play down the significance of the incident.

Indian military officials immediately accepted responsibility for the shooting, but they charged that the Pakistani aircraft had "intruded" at least seven miles inside Indian territory, for several minutes, and had then "acted in a hostile manner" by turning toward the Indian MiG-21 fighter plane that had intercepted it. The MiG then fired an air-to-air missile at the Pakistani plane, Indian officials said.

Each country said the plane's wreckage was in its territory.

Pakistani officials said the plane fell to earth two miles inside Pakistan, and military authorities quickly flew journalists to the crash site to view the wreckage. Pakistani television tonight aired images of border-area swamp, with patches of blood and scattered wreckage visible.

India said its helicopters had found the wreckage a mile inside the border, in an area of empty marshes called the Rann of Kutch. An air force spokesman said the wreckage of the plane had been retrieved and was being brought to New Delhi.

The incident, which killed 11 Pakistani soldiers and five officers, came at a time of extreme tension between India and Pakistan, perennial adversaries that have fought three wars since they became independent nations in 1947. Last year both successfully tested nuclear devices, raising the stakes of their rivalry and altering the security landscape of South Asia.

For eight weeks beginning this spring, Indian troops battled Pakistani-based Muslim guerrillas who invaded a mountainous border area of Indian-controlled Kashmir. Kashmir was divided into Indian and Pakistani sectors in 1948 after both countries claimed the entire region. Since 1989 an armed independence movement has waged a bloody campaign against security forces in India's portion, the state of Jammu and Kashmir.

At least 500 people were killed in this year's conflict, which ended only after President Clinton met with Sharif in early July and pressured him to call off the guerrillas. Pakistan claimed the intruders were Kashmiri insurgents, while India insisted they were Pakistani troops.

Since then, armed insurgent groups have carried out a rash of attacks in the Indian-controlled portion of Kashmir. In the past week, more than a dozen Indian troops have been killed in sabotage attacks in Indian Kashmir. Indian authorities have accused Pakistan of fomenting the assaults, and this week Indian Prime Minister Atal Bihari Vajpayee demanded that Pakistan be declared a rogue state for promoting terrorism.

Indian officials said tonight that the downing of the Pakistani plane would make any resumption of diplomatic negotiations between the countries more difficult. They agreed in March to resolve their long-standing differences peacefully, but the spirit of that agreement was seriously undermined by the subsequent border conflict.

The immediate reaction from both countries has been relatively restrained. According to senior military sources in Pakistan, India's director of military operations called his Pakistani counterpart on a special hot line shortly after the shooting and said India accepted responsibility for it. The sources said the Indian official told Pakistan that the shooting was a "reaction to an intrusion" and that India did not want to exacerbate tensions with Pakistan over the action.

U.N. Secretary General Kofi Annan expressed regret at the loss of life and called on both countries to exercise maximum restraint. In Washington, State Department spokesman James P. Rubin said such incidents "illustrate the need for the two countries to resolve their differences through dialogue." However, he said the United States would play no role for the time being in getting India and Pakistan to talk.

Pakistani officials said the plane--an unarmed, 110-foot-long craft designed for reconnaissance--was regularly used for training missions in the area where it was shot down, and was not used for either reconnaissance or warfare. They said it had taken off from a naval air base 60 miles southeast of Karachi, Pakistan, and was following a flight plan inside Pakistan frequently used in training missions.

But Indian military officials said that the plane strayed well inside India, near the Kori Creek area of Gujarat, and that between May and July there had been eight other intrusions by Pakistani aircraft.

Constable reported from New Delhi; Khan from Karachi.

CAPTION: Officials search wreckage of Pakistani jet India claims entered its airspace.