NAIROBI, Kenya (AP) — Four small children were shot dead while trying to take refuge from the latest al-Shabab extremist attack in Kenya early Tuesday, authorities said.

A Kenyan national police statement said the attackers targeted a telecommunications mast in eastern Garissa county near the border with Somalia, where the al-Qaida-linked extremist group is based.

Police officers on patrol killed two attackers in the early morning assault near the village of Saretho, the statement said. It said a teacher was among the dead.

But a government security report of the incident seen by The Associated Press said four students from the Saretho primary school were killed.

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“The young pupils had taken refuge at the nearby Saretho police post when the attacks began and were guarded by the police when the militants fired at them, killing them instantly and wounding two of their school mates,” the report said.

The report said some attackers escaped with injuries and were being pursued.

Kenyan authorities said bomb-making materials were seized after the attack.

Al-Shabab since December has increased attacks inside Kenya including Sunday’s attack on a military base that killed three U.S. military personnel and destroyed five U.S. aircraft.

It was the first time al-Shabab had attacked U.S. troops inside Kenya. The U.S. on Monday said additional troops had arrived to increase security at the Manda Bay airfield in coastal Lamu county.

The U.S. Africa Command also said it did not assess that the attack was linked to Iran, as Middle East tensions are soaring. Al-Shabab is linked to al-Qaida and has no known links to Iran or its proxies.

Al-Shabab has vowed retribution on Kenya for deploying its troops in 2011 to fight the extremists in neighboring Somalia, where the group is based. Garissa county borders Somalia and Lamu county is nearby.

The extremists often target police as well as civilian targets including buses, schools and infrastructure, mainly in the border region.