With its suicide attack that killed at least 90 people and injured scores Monday, al-Qaeda’s Yemen branch has expanded far outside its sphere of influence in the south, proving it can penetrate even the most sensitive military targets in the capital.

The assault on troops during a parade rehearsal in the heart of Sanaa narrowly missed killing the defense minister and represented Yemen’s most devastating terrorist attack in years. It occurred as militants linked to al-Qaeda defend newly seized territory in the south and confront head-on this Middle Eastern nation’s U.S.-backed government in an intensifying guerrilla conflict.

The Obama administration considers al-Qaeda’s Yemen franchise a key threat to the United States. The branch, known as al-Qaeda in the Arabian Peninsula, or AQAP, has been implicated in a 2009 attempt to bomb an airplane flying to Detroit; a 2010 plot to send parcel bombs to Chicago via cargo planes; and a foiled scheme to send a suicide bomber on a U.S.-bound plane last month.

The United States has dramatically escalated a campaign of airstrikes in Yemen this year, with as many as 21 missile attacks on al-Qaeda elements since January. U.S. Special Operations teams have expanded their role in advising and providing intelligence to the Yemeni military as it launches assaults on militant positions in the south.

In some ways, Monday’s attack was reminiscent of the violent outbursts in Pakistan that accompanied Islamabad’s alignment with the United States against terrorist groups. President Obama alluded to that parallel in remarks he made in Chicago, where he was attending a NATO summit.

Yemen “is attracting a lot of folks that previously might have been in the FATA before we started putting pressure on them there,” Obama said, referring to the Federally Administered Tribal Areas in Pakistan, where al-Qaeda was based. “We’re going to continue to work with the Yemeni government to try to identify AQAP leadership and operations and try to thwart them.”

‘Start of a jihad project ‘

The assailant in Monday’s attack was a soldier participating in a military parade rehearsal in Sanaa’s al-Sabeen Square, security officials said. He blew himself up about 10 a.m. as the troops were listening to the national anthem, minutes before the defense minister was to deliver a speech. The minister and top military commanders are thought to have escaped injury. Most of those killed were soldiers.

In a statement sent to the Reuters news agency, AQAP asserted responsibility, saying the attack was revenge for what it called the U.S. war on its followers in southern Yemen. AQAP said it sought to assassinate the defense minister and other top commanders at the parade rehearsal. The group also warned of more attacks.

“We will take revenge, God willing, and the flames of war will reach you everywhere, and what happened is but the start of a jihad project in defense of honor and sanctities,” said the statement, which was addressed to Yemeni military commanders.

U.S. intelligence officials said that they consider AQAP’s assertion of responsibility credible but that analysts at the CIA and other agencies were still reviewing information about the attack.

Breaching Sanaa’s security

The bombing could reflect a geographical shift in AQAP’s strategy. For months, Sanaa was largely spared from the violence ravaging other parts of Yemen, with the group targeting military and security officials in the south. Radical Islamists have taken over large patches of territory in that part of the country. In recent weeks, Yemeni forces have launched a major offensive against the militants with the help of U.S. trainers, advisers and drone strikes.

Monday’s attack shows that AQAP’s “reach is more pervasive and extensive than people have previously imagined,” said Bruce Hoffman, a terrorism expert at Georgetown University. “If you can pierce the security that most protects the country, that speaks volumes about its existing capabilities and trajectory. It says the group is growing stronger, not weaker.”

Suicide bomber kills or wounds hundreds (Gene Thorp/Digital Globe via Google Earth)

The entrances to Sanaa and virtually every main road within the city are monitored by troops stationed at checkpoints. Monday’s attack raised fears that more AQAP operatives may have infiltrated Yemen’s military, which is divided by political tensions arising from the country’s populist revolt last year. Such infiltration, combined with AQAP’s increasing use of suicide bombers, mirrors tactics used by al-Qaeda in other nations, including Afghanistan, Pakistan, Iraq and, more recently, Syria.

Yemen, the region’s poorest nation, is beset by multiple woes, including rising malnutrition rates and a dire water shortage.

‘It all happened so quickly’

At al-Sabeen Square on Monday, blood and pieces of uniforms covered the road where the soldiers were standing in formation when attack occurred.

“It all happened so quickly,” said Maj. Khaled al-Haimi, a traffic police officer. “We were standing at the end of Sabeen Road when we heard the explosion and saw the soldiers flying in the air.”

Speaking from his hospital bed, injured soldier Majed al-Haj, 22, said, “The explosion happened in the middle of the brigade. We were standing at the end and were injured by the shrapnel. The ones that were in the middle and in the front were killed.”

The military parade was scheduled for Tuesday to commemorate Yemen’s national day, the anniversary of the date in 1990 when North Yemen and South Yemen were unified into one country. President Abed Rabbo Mansour Hadi, who replaced longtime ruler Ali Abdullah Saleh in February, was expected to attend. Since taking office, Hadi has publicly vowed to eradicate the al-Qaeda insurgency. But he is also struggling to contain fighting between loyalists and longtime opponents of Saleh.

After the attack, Hadi vowed to keep fighting AQAP. The militants “wanted to turn the joy of our people with the unity day into sorrow,” he said in a speech to the nation. “The war on terrorism will continue until it is uprooted and defeated completely, whatever the sacrifices are.”

But Obama signaled that such attacks would not serve as provocation for an even wider U.S. role in the Yemeni conflict. The administration has insisted that it is only advising Yemeni forces and would not be drawn into what amounts to a civil war.

“I think one of the things that we’ve learned from the Afghanistan experience is for us to stay focused on the counterterrorism issue, to work with the government, to not overextend ourselves,” Obama said.

Raghavan reported from Nairobi. Staff writer Greg Miller in Washington contributed to this report.