Tens of thousands of police officers will guard churches across the world's most populous Muslim country over the Christmas period, amid concerns that al Qaida-linked militants could carry out attacks, police said Monday. The regional terror network Jemaah Islamiyah has been blamed for annual strikes in Indonesia since 2000, when a series of coordinated bombings killed 19 people on Christmas Eve. An attack on a Christian market last New Year's Eve killed seven. The US Embassy in Jakarta warned of a "serious security threat to Americans and other westerners in Indonesia," saying targets could include hotels, malls, businesses, housing compounds, transportation systems, places of worship, schools or public events. "Terrorists in Indonesia have most often directed attacks at specific buildings or locations. The possibility exists that individual Americans could be targeted for kidnapping or assault," it said in a statement. More than 18,000 police will be posted at thousands of churches and religious sites in the capital, Jakarta, said police spokesman Col. Ketut Untung Yoga. Security was also increased in Poso, on Sulawesi island, which has been the flashpoint of violence between Christians and Muslims in recent years and has seen a surge in tensions since the execution of three Christian militants in September. The measures are a general precaution because leading terror suspect Noordin Top is still at large, Yoga said. "The houses of worship are our priority," he said. The most deadly attack blamed on Jemaah Islamiyah occurred on Bali island in 2002, when simultaneous bombings at nightclubs killed 202 people. That attack was followed by blasts at the J.W. Marriott Hotel in 2003, the Australian Embassy in 2004 and attacks on Bali last year.