LOS ANGELES (AP) _ A Soviet spacecraft made its closest approach to Halley's comet early Thursday, and pictures apparently showing the comet's icy core for the first time were beamed from Moscow to an American TV audience.

The Vega 1 spacecraft was the first of five probes that will sweep by the comet in the next nine days.

''What happened 10 or 20 minutes ago is that the spacecraft made its closest approach to Halley's comet,'' Cornell University astronomer Carl Sagan said from the Soviet Institute for Space Research in Moscow.

The false-color pictures returned by Vega 1 and broadcast in the United States on ABC's ''Nightline'' showed a large blue area, surrounding a green region, which in turn encircled a reddish spot.

''One might surmise that the innermost (reddish) region is actually the icy snowball,'' or nucleus of the comet, said International Halley Watch co-leader Ray Newburn, who watched the pictures from ABC studios in Los Angeles.

''If so, it is the first time we have ever seen the nucleus of a comet as anything other than a point of light,'' Newburn added.

Sagan told ''Nightline'' anchorman Ted Koppel that the reddish spot ''is the presumptive nucleus of Halley's comet.''

The nucleus, which has never been seen, is estimated to be only about 4 miles in diameter and is hidden in the comet's coma.

The comet is now visible from latitudes south of the northernmost United States just before dawn, low above the east-southeast horizon. The comet should be visible as a white blur about a fist's width to the left of the moon.

On Wednesday, the comet was about 106 million miles from Earth and 73 million miles from the sun, traveling at more than 104,000 mph.

The four other probes that will study the comet during the coming days are Japan's Suisei, which will approach within 93,960 miles on Saturday; the Soviet Vega 2 probe, which will fly about 4,986 miles from the comet 18 hours later; Japan's Sakigake spacecraft, which is to study the comet from a distance of 4.3 million miles Monday (Tuesday in Tokyo); and the European Space Agency's Giotto probe.

Giotto is now on a course that will take it within 565 miles of the comet March 13. But a mid-course correction planned for Saturday or later should take the craft within 300 miles of the comet.

Both Vegas and Giotto are equipped with cameras to provide the first close- up pictures of a comet's nucleus. Scientists hope all five probes will also provide clues about the origin of the solar system, since comets are believed to be pristine remnants of the solar nebula - the swirling cloud of dust and gas out of which the planets and the sun formed.