MOGADISHU, Somalia -- Two suicide bombers detonated explosives-laden cars Tuesday outside the office of the UN's mine-clearing agency and a Somali army checkpoint in Mogadishu, killing 13 people, including seven guards, Somali police officials said.

The blasts occurred near the African Union base in the area of the Mogadishu airport, Somali police chief Gen. Mohamed Sheikh Hassan said.

Somalia's Islamic extremist rebels, al-Shabab, claimed responsibility, according to the group's Andalus radio station.

Unlike previous attacks by the al Qaeda-linked al-Shabab, gunmen did not accompany the suicide bomber, said police Capt. Mohamed Hussein. The first suicide bomber tried to speed through the barrier at the UN office but guards shot at the car, he said. The guards were from a private security firm, said a police official, who insisted on anonymity because he isn't authorized to speak to the press.

The deputy spokesman for the UN secretary-general, Farhan Haq, told reporters that if the guards had not stopped the car from entering the UN premises, there "would have been considerably more damage and loss of life."

A second suicide blast targeted a checkpoint manned by Somali security forces near the African Union base in Mogadishu, said Hussein. Casualties there remained unclear.

Al-Shabab is waging an insurgency against Somalia's weak UN-backed government with the goal of establishing an Islamic emirate, ruled by a strict form of Islam.

More than 22,000 peacekeepers serve in the multi-nation African Union force. Al-Shabab opposes the presence of foreign troops in Somalia.

Although al-Shabab was ousted from Mogadishu in 2011, it continues to wage a deadly guerrilla campaign.

"Al-Shabab is desperately seeking relevance and will do anything to keep in the news headlines," the AU Special Representative for Somalia, Ambassador Francisco Caetano Madeira, said in a statement.

Associated Press writer Edie Lederer at the United Nations contributed.