The action followed clashes between demonstrators and police on U.S. campuses and a blockade at Paris's Sciences Po university
The Islamist group also burnt five vehicles in the two incidents, Sheikh Abdiasis Abu Musab said.
"We took all their weapons. There were some Kenyan forces that escaped in the course of the ambush fighting," he said.
Some 20 police died in the ambush, which occurred in Monday evening in Yumbis village, 70 km (45 miles) north of the town of Garissa, the Standard and Daily Nation newspapers said.