Three Navy F14 -fighters chased a group of Libyan warplanes piloted by Syrians away from an Air Force electronic eavesdropping aircraft in a confrontation this week, government sources said yesterday.

It was the second time in less than a week that Libyan fighters harassed an American reconnaissance C135 plane flying in international airspace some 200 miles off Libya, officials asserted.

The C135 was unprotected by fighter escorts in the first incident on Sept. 16. American intercepts of conversations between the pilots of two Libyan planes and their ground control post near Tripoli revealed that they were told to fire missiles at the C135 in that earlier confrontation.

Intercepts indicated that each Libyan plane fired one missile. But the crew of the C135 saw neither missiles nor planes as they patrolled the edge of Libyan airspace, and only heard the conversation, sources said.

Presumably for want of conclusive evidence, the State Department neither disclosed the Sept. 16 incident nor lodged any public protest to Tripoli. However, the Carter administration ordered the Navy to protect the Air Force's eavesdropper C135 on its next mission off Libya.

The C135 took off on Sunday, with one F14 fighter armed with Phoenix air-to-air missiles flying beside it as protector. Libyan fighters scrambled when they saw the radar track of the C135, sources said.

Since ground control sent four Libyan French-built Mirage and four Soviet-supplied Mig fighters aloft, the plan might have been to surround the C135 and force it to land in Libya.

But the Navy had planned a surprise for the Libyan planes this second time around.

As the Libyan planes maneuvered to jump the C135, a military version of the 707 passenger airliner and thus an easy target for supersonic fighters, an additional two Navy F14 fighters launched from the aircraft carrier John F. Kennedy roared out of their hiding places to join the C135.

Faced with three F14s instead of only one, the Libyan planes called their ground commanders for instructions. They were told, according to the intercepted conversations, to break off the engagement.

The intercepts revealed that the radio instructions from the ground to the Libyan planes were in Syrian, confirming for the U.S. government its suspicions that the pilots were Syrian.

All this indicates that tensions between Washington and Tripoli are mounting at the same time President Carter is looking for ways to contain the war between Iran and Iraq.

It is not known whether Carter will risk further conflict in the skies off Libya by continuing to send fighter-escorted C135s on their eavesdropping missions.

Publicly, administration officials are refusing to acknowledge even that the confrontations in the skies have occurred.

Asked about them yesterday at the Pentagon's regular news briefing, spokesman Thomas B. Ross limited himself to "no comment."