A Syrian pilot defected to Jordan on Thursday, flying his MiG-21 fighter jet south across the border and seeking asylum at a Jordanian air base in the first such case involving a plane since the uprising against President Bashar al-Assad’s rule began 15 months ago.

The defection came hours after the U.S. ambassador to Syria, Robert Ford, issued the most direct appeal yet to members of the Syrian security forces to abandon the regime and join the opposition. Although there is no indication that the pilot was responding to the appeal, which was posted on the Facebook page of the U.S. Embassy in Damascus, the defection was welcomed in Washington as a further sign of building discontent within the Syrian military.

“As you know, we have long called for members of the Syrian military to refuse to obey orders, to break with the Assad regime,” said State Department spokeswoman Victoria Nuland. “And we’d like to see more of this.”

The defection came on a day of surging violence across Syria, with human rights groups and activists reporting at least 96 deaths. Most of the deaths resulted from intense shelling of opposition strongholds in the provinces of Homs and Daraa as the army intensified a push to recapture areas that have fallen under rebel control.

In the Facebook posting, Ford issued a stark warning to Assad’s security forces that the United States intends to work with Syrians after the fall of the Damascus government to track down those responsible for the violence and bring them to justice. He reminded them of the strenuous 16-year effort to hunt down and bring to trial the Bosnian Serb military leader Ratko Mladic.

“Members of the Syrian military should reconsider their support for a regime that is losing the battle,” Ford wrote. “The officers and soldiers of the Syrian military have a choice to make. Do they want to expose themselves to criminal prosecution . . .? Or do they want to help secure the role of the professional military in a democratic Syria by supporting the Syrian people?”

Jordan swiftly granted the pilot’s asylum request, a move that risks increasing tensions between Amman and Damascus at a time when the escalating conflict is drawing in regional players. After becoming one of the first Arab leaders to support calls for Assad to step aside last summer, Jordan’s King Abdullah II has since sought to lower his kingdom’s profile in the Syria crisis, amid concerns that Assad’s fall could trigger widespread instability in Jordan.

Defection’s import unclear

It was unclear whether the defection of a lone pilot, identified by Syrian government media as Col. Hassan Mirei al-Hamadeh, signified anything more than an individual case of disgruntlement.

The official Syrian Arab News Agency initially reported that he had gone missing on a training flight. But after Jordan announced that it had granted the pilot’s request for political asylum, the agency denounced him as “a deserter and a traitor to his country” and said he would be “punished accordingly.”

A spokesman for the rebel Free Syrian Army, Col. Malik Kurdi, called Hamadeh a hero “who shared in the suffering of the Syrian people and expressed his rejection of the tyranny practiced by the regime.”

Speaking by telephone from the Free Syrian Army’s de facto headquarters in a refugee camp in southern Turkey, Kurdi said that Hamadeh was from the mostly Sunni province of Idlib, a rebel stronghold, and that many more pilots would like to defect, “but there are strict measures and a lot of controls on them.”

The Syrian air force has largely sat on the sidelines as Assad’s army has acted to crush the revolt, making the pilot’s flight to Jordan less significant than the high-profile defections of a small number of Libyan pilots in the early days of that uprising last year. Then, Libyan leader Moammar Gaddafi’s immediate deployment of aircraft resulted in the swift imposition of a NATO no-fly zone.

A largely civilian rebel force

But Hamadeh’s flight underscored the steady trickle of defections occurring daily across Syria, usually in small groups numbering no more than a few dozen. Although limited, those defections have been sapping the strength of the security forces while helping to shore up the rebels.

Military experts and diplomats believe that the majority of the tens of thousands who are estimated to have deserted their units simply discard their uniforms and go home. But some join the Free Syrian Army, and they provide the backbone, if not the bulk, of a rebel force in which armed civilians are thought to outnumber defectors.

Kurdi put the number of deserters at 60,000 to 70,000 and said that about 40 percent of them join the Free Syrian Army.

Joseph Holliday, who closely monitors Free Syrian Army activity at the Washington-based Institute for the Study of War, put the total size of the rebel force at 40,000 and said he suspects that most members are civilians.

There has not been a high-ranking or large-scale defection significant enough to strike a blow against the dominance of the estimated 250,000-strong Syrian army over the lightly equipped rebels. The vast majority of the defections have come from the rank and file of the conscript army, with only a few generals reported to have abandoned their posts, and no key members of Assad’s inner sanctum.

However, as the rebels apparently gain strength, aided by an influx of new weaponry funded by Saudi Arabia and Qatar in coordination with the United States, the strain on the already stretched army seems likely to grow.

“The operational tempo is going to increase,” said Jeffrey White, a defense fellow at the Washington Institute for Near East Policy. Syrian troops are “seeing their compatriots killed in combat, and this is going to lead to more defections and desertions. . . . The regime is under some substantial pressure in a lot of places.”

Staff writer Karen DeYoung in Washington contributed to this report.