ADEN, Yemen (AP) A suicide car bomber struck Yemen's Defense Ministry Thursday, killing 18 soldiers and paving the way for a carload of gunmen wearing army uniforms to storm the heavily guarded compound in the capital of Sanaa, officials said.

The brazen morning attack underlined the ability of insurgents to strike at the heart of the government as they exploit the instability that has plagued the U.S.-allied country for more than two years. Yemen's Defense Minister Mohammed Nasser Ahmed was in Washington on Thursday for talks with U.S. officials.

There was no immediate claim of responsibility for the attack, but suicide bombings and complex attacks are the hallmarks of al-Qaida.

The U.S. considers Yemen's al-Qaida branch, known as al-Qaida in the Arabian Peninsula, to be the world's most active and has sharply escalated drone attack targeting the militants. American forces also have been training and arming Yemeni special forces, and exchanging intelligence with the central government.

The terror network gained a major foothold in the south, taking over several towns amid the chaos that followed the 2011 uprising that ousted President Ali Abdullah Saleh. The drone strikes and a series of U.S.-backed military offensives helped uproot several key militant strongholds, but al-Qaida continues to fight back.

Hospital and military officials, who spoke on condition of anonymity because they weren't authorized to release the information, gave the death toll and said 40 soldiers also were wounded.

The Defense Ministry said "most" of the gunmen who were armed with assault rifles and grenades also were killed but did not give a number or more details.

President Abed Rabbo Mansour Hadi, who succeeded Saleh, later met with military commanders inside the devastated complex. He also ordered an investigation into the incident, military officials said.

The officials said the blast badly damaged a hospital inside the complex, started a fire and blew out windows and the doors of nearby homes and office. The blast and the subsequent gunfight destroyed an armored vehicle belonging to the army and reduced three civilian cars outside the complex to charred skeletons, witnesses said.

Video footage of the aftermath showed a vehicle engulfed in flames as soldiers and ambulances arrived at the ministry. Gunfire echoed in the streets and sirens wailed.

Military helicopters hovered over the site and state television aired calls for blood donations.

Al-Qaida militants are concentered in the southern and eastern parts of Yemen, but they occasionally strike in the capital.

Yemen is strategically located at the southwest corner of the Arabian Peninsula, bordering Oman and Saudi Arabia, two of Washington's closest Arab allies. Yemen has a shoreline on the Red Sea and the Arabian Sea close to the vital shipping lines carrying oil from the energy-rich Gulf region to the West.