SURABAYA, Indonesia, May 14 (Reuters) - A vehicle exploded injuring several Indonesian police in Surabaya on Monday, a day after Islamist militants killed at least 13 people in suicide attacks on churches in the country's second largest city, police said.

The blast had occurred at 8.50 a.m. (0150 GMT) at their police office, East Java police spokesman Frans Barung Mangera told a briefing. The full extent of casualties was unclear, he said.

"There has been an explosion, we don’t know exactly what happened," he said. On Sunday, Islamist suicide bombers attacked three churches, killing at least 13 people and wounding about 40. (Reporting by Kanupriya Kapoor Writing by Ed Davies; Editing by Simon Cameron-Moore)