An Israeli soldier and taxi driver were being held in Jordan last night after making what border authorities said was an "accidental" crossing over the Jordan River.

Israel asked the International Red Cross to begin negotiations for the return of the two men, who were seized by Jordanian soldiers after crossng the Allenby Bridge, the most heavily traveled span between Jordan and Israel.

There were conflicting accounts of what happende, but Israeli officials denied that the taxi intentionally crossed into Jordanian territory, or that the occupants defected to Jordan. They described the incident as a "fluke," and demanded an early return of the two men.

According to Israeli army officials, a commercial taxi was ordered in Jerusalem to meet tourists at the Allenby Bridge, about five miles east of Jericho. Several hundred thousand vehicles have crossed the span each year since Israel seized the West Bank in the 1967 six-day war.

Army officials said the taxi passed one checkpoint about a mile from the bridge, and received clearance to proceed to a terminal on the Israeli side about a half mile from the river. There, the taxi driver, whose name was withheld, passed through another border checkpoint to go to the edge of the narrow, 40-foot bridge to meet the expected passengers.

Officials said a uniformed civil guard soldier accompanied the taxi driver to the edge of the river for the final clearance.

An army spokesman in the Tel Aviv Defense Ministry said that at that point, the driver asked a bridge guard to let him pass so that he could turn his taxi around for the return trip to Jerusalem. Instead, the spokesman said, the driver drove onto the span, apparently intending to turn around on the other side.

The bridge is too narrow for the limousine-sized taxi to turn, he said. After the taxi crossed the halfway point, Jordanian soldiers siezed it and its occupants and took them away at gunpoint, officials said.

However, another Israeli military spokesman said the taxi simply drove to the bridge, and instead of stopping as instructed, drove across. "There was nothing we could do. We were astonished, and because both of them are on the other side, we don't know the reason," the official said.

Normally, Israeli and Jordanian soldiers are stationed at the bridge's midpoint, and often they spend their time chatting with one another. There was no explanation of how the taxi passed unimpeded through both security lines.