The defection of four senior Syrian military officers to the Syrian opposition this week is another sign that senior officials are turning away from the government of President Bashar al-Assad, the United States said Friday.

"We have had four senior Army officers -- two brigadier generals and two colonels -- defect yesterday and join the opposition," State Department spokeswoman Victoria Nuland told reporters. "We have been calling for many, many weeks for members of the military to vote with their consciences and to break ties with Assad, and to refuse orders and to refuse to participate in the violence that's ongoing. So we are beginning to see this stream accelerate, and that's a good thing."

The defections came the same week a Syrian pilot landed his military jet in neighboring Jordan and announced his defection. Nuland said U.S. officials had not yet been in contact with the Syrian pilot, but were in contact with Jordanian authorities.

While there's no sign of collapse by Assad's most elite military units, the rank and file may be less loyal. Opposition sources say that some Syrian troops are deliberately missing their targets.

"It stands to reason that if there are desertions from the ranks of the Syrian military there's disgruntlement in the ranks," a U.S. official said. "So far, it looks like a phenomenon limited mostly to Sunnis and to lower-ranking officers. It remains to be seen whether the Syrian pilot's request for asylum in Jordan will touch off similar actions."

U.S. officials say opposition forces are strong enough that Assad's most elite units cannot always respond everywhere they are needed.

"We have an opposition -- a set of opposition groups that is finding ways -- they're not totally coalesced, but they are finding ways of organizing themselves more effectively," Pentagon spokesman George Little said at a press conference Thursday.

In addition to the defections, a senior U.S. official with access to the latest information said there's also an increasing supply of weapons from the Syrian military itself.

"They just purchase them from officers, the Syrian system is quite corrupt," said Andrew Tabler, an expert on Syria at the Washington Institute and author of "Lion's Den: An Eyewitness Account of Washington's Battle with Syria." "Many times when people are stopped at checkpoints, soldiers ask if they'd like to buy any ammunition."

On his Facebook page earlier this week, Robert Ford, the U.S. ambassador to Syria, directed a post to members of the Syrian military, telling them they should "reconsider their support for a regime that is losing the battle."

Ford pointed out that the international tribunal for the former Yugoslavia prosecuted military officers for their roles in the Balkan conflicts of the 1990s, and suggested a similar process in Syria.

"I want to make it clear that the United States and the international community will work with the Syrian people to locate the military members responsible for this violence and hold them accountable," Ford wrote. "And we will support the future Syrian government's efforts to bring those people to justice.

"Soldiers should know that, under international law, they have a responsibility to uphold basic human rights and that they do not escape responsibility for violations simply because they are subject to orders."

Ford has been working out of the State Department in Washington since the United States shuttered its embassy in Syria and pulled out its remaining staff in February after the government there refused to address its security concerns.

In her briefing Friday, Nuland said there have been no reports at this stage of any senior officials in al-Assad's government defecting, but there are signs that some officials may be rethinking the longevity of al-Assad's rule.

"We have seen plenty of money flowing out of Syria," Nuland said. "We've seen plenty of family members moving out of Syria, and these are often good indications of how people feel about the staying power of the government."