Al-Qaida-linked militants detonated multiple bomb blasts and engaged in ongoing battles with security forces in an attempt to breach the main U.N. compound in Mogadishu, officials said Wednesday.

Somali police officer Abdi Hassan said gunmen had breached the compound and were inside, while the militant group al-Shabab said on its Twitter feed shortly after the attack that its fighters "are now in control of the entire compound and the battle is still ongoing."

A U.N. official said that African Union troops and Somali security forces were responding to the attack. The official said the senior U.N. representative was not at the compound during the attack and is safe at the airport.

The U.N. was trying to figure out the status of its other workers, though. One U.N. official in Mogadishu and in contact with people inside the besieged compound said the gunmen had not been able to get inside. The official said there had been no casualty reports among U.N. staff but that some guards had been wounded.

"We were actually holding a meeting close to the compound when we heard the explosion," the official said.

A second U.N. official said the staff inside the building had been moved to bunkers and he suggested the staff wouldn't actually know if the gunmen were inside the compound or not. Both U.N. officials insisted on anonymity because they are not authorized to be quoted by name.

"All I know is that it's sustained fire and they're inside so they don't know what's going inside," said one U.N. official. "We have bunkers so all staff have been moved to the bunkers where it's safe."

The official said the staff had described "several" explosions. An Associated Press reporter at the scene said one of those blasts included a car bomb.

Al-Shabab said that members of its martyrdom brigade were carrying out the attack.

The compound under attack lies just across the street from the secure airport complex, where African Union military forces are based. The U.N. compound is used by agencies like UNICEF, WHO and UNDP.

The top U.N. official on Somalia, Nicholas Kay, also works out of the building. He was not inside the compound when it was attacked and is safe inside the airport compound, one of the U.N. officials said.

Mogadishu fell into anarchy in 1991 and is just beginning to move past years of sustained conflict. The U.N. and foreign embassies were absent from Mogadishu for close to two decades.

But African Union forces pushed al-Shabab out of Mogadishu in August 2011, meaning residents didn't have to live through daily battles for the first time in years. An international presence slowly began to return.

U.N. Secretary-General Ban Ki-moon visited Mogadishu last December and announced that the U.N. would re-open its offices in the seaside capital. The U.N. began the process of moving its personnel from the nearby capital of Nairobi, Kenya, back to Mogadishu, a process that has accelerated in recent weeks.

International embassies _ from Turkey and Britain, for example _ followed. Wednesday's attack, though, underscores the fragile security situation and will force the U.N. and embassies to review their safety plans and decide if they have enough defenses to withstand a sustained al-Shabab assault.