When pro-government snipers killed 52 protesters last month, opponents of Yemeni President Ali Abdullah Saleh thought the United States would finally support calls for his immediate resignation.

But the Obama administration has yet to publicly urge one of its key allies in the Middle East to step down. Its policy has fallen far short of the strong actions that Washington took in Egypt and in Libya, where it is using its military and diplomatic might to press for a new leadership. Since that violent March day, the attacks on protesters have continued.

Now, many activists here are convinced the United States will speak up only when the death toll rises significantly. “We feel disappointed, and we feel left out,” said Khaled Al-Anesi, a human rights lawyer and activist.

Anger and frustration at the Obama administration are building among activists and political opposition leaders amid intensifying violence across this Middle Eastern nation. As Saleh remains defiant, many here believe the United States should apply more pressure to bring his 32-year rule to a swift end.

“The American administration has been too slow to act,” Anesi continued. “Until now, we have not seen a firm public stance. They don’t have to wait until thousands of us are killed.”

The crisis in Yemen, perhaps more than elsewhere, underscores the dilemma the Obama administration faces in grappling with the ongoing rebellions across the Middle East and North Africa and their potential consequences. On one hand, Yemen’s activists are fighting for democratic values that Obama strongly signaled he would support in his Cairo speech in June 2009. On the other hand, Saleh is a vital U.S. ally in a nation that poses the most significant terrorism and security threat to the United States after Afghanistan and Pakistan.

An oil-producing region nestled along strategic shipping lanes, Yemen is home to al-Qaeda in the Arabian Peninsula. The al-Qaeda affiliate, known as AQAP, has tried to attack the United States twice over the past two years, with a failed plot to bomb a U.S. airliner over Detroit on Christmas Day in 2009 and last year’s attempt to send parcel bombs to Chicago.

For several weeks, the United States has been pushing behind closed doors for a peaceful transfer of power and has denounced the violence in Yemen. But publicly the administration continues to send mixed signals. The Pentagon still expresses doubts that a post-Saleh government can effectively combat AQAP, terrorism and potential influence from Iran; the State Department has yet to publicly demand regime change, even though in private U.S. officials acknowledge that Saleh is no longer capable of leading Yemen.

Saleh’s opponents say he has exaggerated the threat of al-Qaeda to help persuade the United States to support him. The absence of a strong U.S. stance against him, they say, has allowed Saleh to use his close relationship with the United States as leverage in power transfer talks, as he argues that Washington wants a gradual transition of power and that he should oversee such a change. The more he prolongs his departure, the more Yemen inches toward violent upheaval, even civil war, they warn.

“The international community needs to freeze his accounts and stop all means of support for him,” said Tawakkol Karman, a key architect of the uprising. “They need to deal with him as an illegal regime.”

Diplomats and Yemeni officials have long warned the Obama administration of Saleh’s vulnerabilities. Yet Washington has continued to back Saleh, seeing little alternative to tackling AQAP. Last year alone, the United States gave $300 million in military and development aid, largely to fight AQAP and contain its reach.

What the Obama administration worries about most in the face of Saleh’s possible departure is who will replace him. Decades of marginalizing his opponents has left behind a weak and divided opposition, raising concerns about its capability to fight AQAP.

Last month, U.S. Defense Secretary Robert M. Gates appeared to suggest that the administration needed Saleh, saying that the United States “had a lot of counter-terrorism cooperation” with Saleh and Yemen’s security services. He added that “if the government collapses or is replaced by one that is dramatically more weak,” then the United States would face “additional challenges out of Yemen.”

Last week, though, State Department spokesman Mark Toner told reporters that the United States did not solely depend on Saleh to fight AQAP, saying that “our shared interests in fighting counter-terrorism extends beyond one individual.”

But when asked why the United States was not explicitly calling for Saleh to step down when his security forces are killing his own people, Toner said: “This is something that the Yemeni people need to dictate and demand and that he needs to respond to their aspirations. It’s not for us to impose a solution.”

The Obama administration’s dilemma in calling for regime change in some states, but not others, was illustrated in a recent speech by Secretary of State Hillary Rodham Clinton in Washington. Clinton congratulated the people of Tunisia and Egypt, where entrenched leaders were ousted, and repeated the U.S. insistence that Moammar Gaddafi move aside in Libya.

But she tempered her remarks on those countries where turmoil threatens U.S. interests. In Bahrain, she called for an unspecified “political process” and warned against violence.

In Yemen, Clinton said, the United States strongly supports the “quest for greater opportunity” and the “pursuit of political and economic reform.” Saleh, Clinton said, “needs to resolve the political impasse with the opposition so that meaningful political change can take place in the near term in an orderly and peaceful manner.”

In interviews, many activists said they believe in free and fair elections, separation of powers, freedom of speech and other core democratic values — so they expected the United States to take a strong public stance against Saleh.

If this doesn’t happen, Karman warned, the protesters would themselves ratchet up their rebellion, knowing full well they could invite the wrath of Saleh and his security forces.

“We are willing to pay with our blood if takes that to reach our goals,” she said. “And we are running out of patience.”

Staff writer Karen DeYoung in Washington contributed to this report.