US, Nicaragua tensions rise. Washington and the CIA deny any connection to the American plane downed by Nicaragua. But Managua's claim that it captured a US military adviser raises tensions before Congress finally approves contra aid. Stories below, P. 3

The Nicaraguan government's claim to have captured a United States airplane bringing supplies to the Nicaraguan rebel guerrillas nudges hostilities between Managua and Washington a dangerous notch higher, diplomats here warn. Should Eugene Hafenfuf, as the Sandinista authorities identified their prisoner, indeed prove to be the US military adviser in El Salvador that Managua says he is, another piece in the jigsaw of US covert operations against Nicaragua will have fallen into place.

``Anything that amounts to foreign involvement must raise the tensions,'' a Western envoy says. He cautions, however, ``We must wait for the Americans' version if he [Hafenfuf] is indeed an American.''

Nicaragua announced Monday night that its troops had downed a camouflage transport plane 36 hours earlier in southern Nicaragua, killing three Americans aboard and capturing a fourth.

Observers here find it hard to believe that the US would risk sending an identifiable serviceman on an illegal mission into Nicaragua only days before the US Congress is expected to finally approve $100 million in US aid to the contra rebels.

Whether or not Mr. Hafenfuf has connections with the United States government, he is still the first American prisoner of war to have been captured in the United State's 4-year proxy war against Nicaragua. This could provide a further point of friction between Managua and Washington.

As a prisoner of war, Hafenfuf could be liable for trial in a Nicaraguan court. Nicaraguan law provides for a maximum jail sentence of 30 years.

The downed airplane, the Nicaraguan authorities say, had been ferrying military supplies from neighboring Costa Rica to the US-backed contra guerrillas in southern Nicaragua.

After a preliminary questioning of Hafenfuf, the Sandinistas identified him as a 35-year-old ``military adviser in El Salvador.''

They did not clarify, however, whether he was serving with the United States military, the Central Intelligence Agency, or a mercenary organization. Officially, the United States maintains 55 military advisers in El Salvador, working with the Salvadorean Army in its nearly 7-year war against the leftist guerrillas. Independent US groups, such as Soldier of Fortune and Civilian Mat'eriel Assistance (CMA), however, have also sent free-lance advisers to assist Salvadoreans.

Monday's announcement recalled the deaths of three other US citizens brought down by Sandinista antiaircraft fire.

Warrant Officer Jeffrey Schwab died in January 1984 when his US Army helicopter was forced down just inside Honduras after overflying Nicaraguan territory.

Eight months later, two members of the independent Civilian Mat'eriel Assistance group died in a helicopter crash as they accompanied a squad of contras on a mission over northern Nicaragua.

Under current United States law no United States official is allowed to work with the contras on any military assignment. Under the contra aid bill, expected to become law within days, United States military and Central Intelligent Agency operatives may train the contras so long as they do not come closer than 20 miles to the Nicaraguan border.

The Nicaraguan claim that their prisoner was based in El Salvador also draws fresh attention to the role that US allies in the region -- Honduras, El Salvador, Costa Rica -- have been playing in Washington's campaign against the Sandinistas.

The presence of thousands of Nicaraguan contra rebels in Honduras, close to Nicaragua's northern border, is well known although still officially denied by the Honduran government. Nicaragua's southern neighbor, neutral Costa Rica, is also host to Nicaraguan contra leaders and several hundred of their troops.

But less clear is the Salvadorean contribution to the contras' logistical and offensive operations.

Nicaraguan contra military leaders, including the former chief of the Democratic Revolutionary Alliance, Ed'en Pastora G'omez, are known to have visited the Salvadorean Air Force headquarters at Ilopango, outside San Salvador.

Supply flights for the Nicaraguan contra rebels reportedly take off from the base regularly. And two Cuban exiles captured with contra forces early this summer told reporters that they had stopped off at Ilopango, en route to Costa Rica, after they had been recruited into rebel ranks in Miami.