ISLAMABAD, Pakistan (AP) _ A abandoned bomb in a bag exploded on a passenger bus Sunday, officials said. Up to nine people were injured.

No group claimed responsibility for the attack in this capital city. Police blamed terrorists but gave no details.

Police said the bag was placed under a seat at the rear of an intercity bus and the explosion blew out windows and doors.

Dr. Humayun Rizwan, reached at a local hospital, said nine people were injured. However, police said only four people were hurt.

Authorities have blamed scores of bombings in Pakistan over the past two years on agents of Afghanistan's communist government.

Pakistan arms and supports Moslem guerrillas fighting the Kabul government, which is backed by Soviet troops.

The U.S. State Department reports Pakistan was the leading target of terrorist activity during the past year. The country is home to more than 3 million Afghan refugees.

In the past, Islamabad has rarely been a target, but Sunday's blast was the second there in less than a month. The earlier explosion was small and caused no injuries or damage.

Security has been tightened in the capital over the past year, with more police patrolling and checkposts set up around the clock on roads leading into the city.