TRIPOLI, Libya (AP) — A massive truck bomb exploded near a police base in the western Libyan town of Zliten on Thursday, killing at least 60 policemen and wounding around 200 others, officials said.

No group immediately claimed responsibility for the attack but a local Islamic State affiliate has been trying to gain a foothold in Zliten, spreading westward from its central stronghold in the city of Sirte along the North African country’s coast.

The U.N. special envoy to Libya, Martin Kobler, denounced the attack and urged Libyans to “put their differences aside and unite to confront the scourge of terrorism.” U.N. Secretary-General Ban Ki-moon condemned the attack as well as ongoing attacks by the Islamic State group on oil facilities near Sidra and called for a national unity government as “the best way for Libyans to confront terrorism in all its forms.”

The bombing was yet another reminder for Libyans that “urgent progress is required” toward empowering a new unity government and rebuilding state bodies, Kobler said in a statement.

Hours after the blast, rescue crews at the scene had only managed to extract 60 bodies out of the wreckage, said a hospital spokesman, Moamar Kaddi. Libyan officials said they believed there might be dozens more dead.

The police base, where about 400 recruits were training, was used by Libya’s border police, a Zliten security official said. Border police foiled numerous human smuggling attempts off the coast of Zliten last year. The official spoke on condition of anonymity because he was not authorized to speak to the media.

In Washington, White House spokesman Josh Earnest said Thursday that the U.S. has not yet determined who is responsible for carrying out a “cowardly act of terrorism” and extended condolences to the victims and the families of those who were killed, and to the Libyan people. Earnest said the U.S. remains “deeply concerned” about Islamic State-inspired militants carrying out acts of violence in Libya.

Smugglers operating in Libya are notorious for responding with violence to any attempt to disrupt their lucrative operations, but there have been no reported incidents in which they used car bombs, suggesting that Islamic militant are more likely to have been behind Thursday’s attack. Also, it was not immediately clear whether the attack was a suicide bombing, a hallmark method of Islamic militants.

In recent years, thousands of migrants seeking a better life in Europe sailed from Libya on rickety, overcrowded boats. Hundreds have drowned in those crossings.

Libya slid into chaos following the 2011 toppling and killing of longtime dictator Moammar Gadhafi. The oil-rich country is torn between an Islamist government based in the capital, Tripoli, and a rival, internationally recognized administration in the east. Meanwhile, a U.N.-supported unity government sits in neighboring Tunisia.

Residents in Libyan coastal cities have long expressed fears of the variety of smugglers and traffickers who run lucrative operations along the Mediterranean Sea. Authorities have echoed the same concerns, claiming they are unable to fully tackle these networks without international assistance.