The White House is under pressure to ramp up counterterrorism action against al-Shabaab in Somalia following the al-Qaeda-linked group's deadly attack on an upscale Kenyan shopping mall that has killed and injured dozens, including Americans.

Republican lawmakers Sunday said the attack showed al-Qaeda is growing in size and strength, belying the Obama administration's claims that it has grown weaker.

"They're not on the decline," said Sen. Tom Coburn, the ranking Republican on the Homeland Security Committee, on CBS' "Face the Nation." "They're on the rise, as you can see from Nairobi."

Al-Shabaab militants launched their assault on Saturday, storming the mall with grenades and gunfire. Kenyan security forces launched a "major" assault late Sunday on the mall, where the militants were still holding an unknown number of hostages, trying to end the two-day standoff that had already killed at least 68 people. The Kenya Defence Forces said their troops had rescued "most" hostages and taken control of most of the mall in Nairobi.

However, that could not be confirmed and a person connected to the situation told The Associated Press on Monday the hostages have not been freed.

No Americans killed

State Department spokesman Marie Harf said five U.S. citizens were among the more than 175 injured, but no Americans were among those reported killed. Harf said U.S. law enforcement, military and civilian personnel in Nairobi are providing advice and assistance as requested by the Kenyan authorities.

U.S. counterterrorism officials throughout the Obama administration have debated whether to target the Somalia-based rebel group more directly, especially after it merged with al-Qaeda in early 2012. But U.S. action has been limited to the occasional drone strike or raid when a particularly high-value al-Qaeda target comes into view, while relying primarily on assisting Somali and African peacekeeping forces to carry out the day-to-day fight.

That decision was partly driven by the fear that directly targeting al-Shabaab would spur the group to expand its own target list, striking at U.S. diplomatic posts overseas and calling on members of the Somali diaspora inside the U.S. to carry out attacks, according to multiple current and former U.S. counterterrorism officials. They all spoke on condition of anonymity because they were not authorized to discuss publicly internal policy decisions.

A White House official said Sunday that the administration had taken a "balanced approach."

"It's not a question of either direct action or playing a supporting role," National Security Council spokesman Jonathan Lalley said by email Sunday. "Our approach has been to work to enable and support African partners," as well as prosecuting some al-Shabaab members and supporters, he said.

Drone strikes

"The U.S. military has also taken direct action in Somalia against members of al-Qaeda — some of them members of al-Shabaab — engaged in efforts to carry out terrorist attacks against the United States and U.S. interests," Lalley said.

But that effort in Somalia pales next to, say, the hundreds of U.S. drone strikes against militants in Yemen and Pakistan during the Obama administration.

The Somali rebel group has similarly limited its own target list to Somali officials or troops, and African Union peacekeeping troops, to avoid drawing the U.S. counterterrorism machine into a full-fledged fight, the U.S. officials say. Though headed by hard-core Islamist militants, al-Shabaab's more moderate membership has successfully argued to keep the group focused on overthrowing the U.S.-backed Somali government, rather than taking on the mantle of al-Qaeda's larger war with the west.

The group did claim responsibility for twin suicide bombings in Uganda in 2010 that killed more than 70 people, but that was seen as a reaction to Uganda providing the bulk of African peacekeeping forces in Somalia.

Similarly, al-Shabaab said this weekend's attack was in retribution for Kenyan forces' 2011 push into Somalia.

But the scale and technical sophistication of the Nairobi attack could signal a change in al-Shabaab's aspirations, according to Rep. Peter King, possibly increasing the group's direct threat to the United States.

Recruiting in U.S.

King, a Republican who serves on the House Intelligence Committee, said the State Department had not initially wanted to declare al-Shabaab a terrorist organization because it saw the group focusing on tribal issues within Somalia. It was declared a terror organization in 2008.

"We're talking about very significant terrorist groups here which are showing a capacity to attack outside of their borders and actually recruit people from here in the United States," King said Sunday on ABC's "This Week."

Up until now, President Barack Obama secretly has authorized only two commando raids and at least two drone strikes against the al-Qaeda linked terrorists in Somalia, while a small U.S. special operations team has advised African peacekeeping troops, as well as helping build a small elite Somali counterterrorism force, according to two former U.S. military officials familiar with the operations.

Two former U.S. counterterrorism officials say the preference has always been to meet specific incidents with a specific response but to avoid getting too deeply involved in the continent of Africa.

They said the number of western citizens among the dead and injured in the weekend incident may change the U.S. calculation