Tall and slim, blow-dried even in the rugged military camp, the long-legged operative -- known as "the crane" -- was a welcome symbol of Nicaraguan womanhood for anti-Sandinista guerrillas enjoying a respite from the ugliness of battle.

But to leaders of the rebel Nicaraguan Democratic Force, the beauty with the purple eye shadow also was a reminder of a broad underground conflict that has arisen during the past three years, with the Honduran-Nicaraguan border hills as its main arena.

"The crane" was either a recruit to the rebels' battle against the government in Managua, the leaders told visitors to the camp, or another infiltrator sent as a spy by the Sandinista General Directorate of State Security.

As the U.S.-backed contra, or counterrevolutinary, guerrillas have expanded their fight to overthrow the Sandinista government, so has the Sandinista security apparatus expanded its efforts to penetrate the rebel group and obtain information on its plans and activities in Honduras, guerrilla leaders report.

According to Honduran and U.S. officials, the Sandinista shadow war also has included several attempts to promote subversion within Honduras by training and supplying leftist Honduran revolutionaries seeking to establish an underground movement to overthrow the Tegucigalpa government.

Honduran and U.S. officials have accused the Sandinistas of starting support for Honduran rebels even before anti-Sandinista forces began to organize here in 1981 with help from the Honduran Army and the CIA.

The Reagan administration cited what it called the latest sign of such support -- capture of seven Nicaraguan agents in southern Honduras -- in announcing its economic embargo against the Sandinistas to punish "aggressive activities in Central America."

Whatever the start-up date, subterranean Sandinista activities here appear by now to be part of a multilateral struggle in which the United States is also involved and whose most visible element is the so-called "covert" war being waged by the Nicaraguan Democratic Force from bases in Honduras, with vociferous encouragement from Washington.

According to U.S. officials citing Honduran intelligence reports, the seven Nicaraguans captured in mid-April had infiltrated into southeastern El Paraiso province with a double mission: to supply arms and other equipment for a leftist Honduran guerrilla network, but also to encourage the Honduran leftists to attack anti-Sandinista guerrillas. Their central camp lies in the area where the seven were apprehended.

A Honduran source who monitors leftist activity here expressed skepticism at the administration reports emphasizing help to leftist guerrillas. Sending Nicaraguan agents to foment a Honduran guerrilla network would be an unlikely tactic, he said.

Several hundred Honduran leftists from five different groups have been reported in exile in Nicaragua or Cuba seeking help to build such networks, he said. They, not Nicaraguans, would be the obvious candidates for infiltration into Honduras, the source said.

The more likely purpose of the Nicaraguans' mission here, he added, was to enlist Hondurans to help them sabotage or infiltrate the contras' camp, at Las Vegas in southeastern El Paraiso.

The Honduran Army, which is holding the captured Nicaraguans, declines to explain their mission, confining itself to confirming reports from Washington and saying interrogation is continuing.

Many Nicaraguans who have said they crossed the border fleeing military draft have been similarly interrogated in recent months. Honduran officers suspect that Managua has dispatched many agents among the scores of Nicaraguan youths entering Honduras every month, some of whom end up in the ranks of anti-Sandinista rebel forces.

Despite the attention focused on large-scale U.S. military maneuvers and U.S. complaints about Nicaraguan armor and troop buildups, secret Sandinista support for Honduran subversives has been regarded by many military officers here as the major threat to their country. Reflecting these concerns, U.S. Special Forces teams from Fort Bragg, N.C., and Panama have held frequent counterinsurgency exercises in Honduras during the past year, often unannounced.

Helping the anti-Sandinista rebels, say some Honduran officers, increases chances of Sandinista retaliation through subversion in Honduras, particularly if Honduras cannot rely on U.S. backing.

Rebel officials have acknowledged Sandinista infiltration, saying they, too, have their own plants in the Sandinista government.

The way "the crane" tells it, she was sent to infiltrate the rebel leadership last year to provide intelligence to the Sandinistas, but decided to remain with the rebels. According to her story, this was her second change of mind.

The young woman told reporters that she started as a courier with a clandestine anti-Sandinista network inside Nicaragua. As part of her work, she said, she met and eventually became the mistress of a Nicaraguan known only as "the fish."

Although he claimed to be part of the network, she recounted, "the fish" turned out to a Sandinista spy. Rebels said he came to Tegucigalpa posing as an underground anti-Sandinista operative, obtained a list of clandestine rebel sympathizers and went back to Nicaragua to denounce them all -- including his mistress.

In an extensive interrogation by Sandinista security agents, "the crane" said, she was persuaded to switch sides and go to Honduras to join the Democratic Force as a spy. But once inside the group here, she said with its officers looking on, she decided that the rebels were right after all and told them of her mission.

Col. Enrique Bermudez, the force's commander, said the young woman has been allowed to stay in the main rebel camp while the leadership tries to figure out which of her loyalties is authentic.