An Air Force EC135 electronic eavesdropping plane flying at the edge of Libyan airspace may have been fired upon Monday by Libyan fighter planes, government sources said yesterday.

The reason the Carter administration is not protesting the suspected attack, sources said, is that the evidence is fragmentary. The American pilots apparently intercepted talk between two Libyan fighter pilots that each had fired a missile at the C135.

The American pilots saw neither the Libyan planes nor the missiles, sources said. The missiles missed their mark, if they were fired. Government sources said the encounter took place along the edge of Libyan airspace, where such electronic snooping is usually done.

In this age where airplane radars can see much farther than pilots, the hunter and hunted can do battle in the skies without ever seeing each other. The F15 fighter's radar, for example, can see 100 miles. Life and death in such long-distance missile battles often depends on reading warning lights and green lines on radar scopes in time.

If the Libyan pilots did fire their missiles, it could mean that Libyan leader Col. Muammar Qaddaff is stepping up his challenge to American planes and ships coming near his territory.

Qaddafi in the past has protested American military intrusion in his region. It took a hard line against the United States on Sept. 1, when he celebrated the 11th anniversary of the revolution that brought him to power. "We are on the front lines of the struggle against Israel and the United States," he said.

However, Qaddafi's country still sells oil to the United States.

North Korea, after protesting Americana reconnaissance activities, hijacked the spy ship Pueblo in 1968 and shot down a Navy EC121 electronic eavesdropping plane in 1969. All 31 crew members of the plane were killed when the EC121 crashed into the Sea of Japan off North Korea.

Big, slow-flying planes like the C135 and EC121 are easy marks for modern fighters and their radar-directed and heat-seeking missiles. It could be that the Libyan pilots were simply trying to bluff the Air Force EC135 out of their area by talking about downing it, sources said.