Security forces have been struggling to contain the violence in Somalia
Nine policemen have been killed in an attack on a police station in the Somali capital, Mogadishu.
Al-Shabab, a radical wing of the Islamist Alliance for the Re-Liberation of Somalia, claimed responsibility.
The alliance signed a three-month ceasefire pact with the government earlier this month.
But a prominent Islamist leader, Sheikh Hassan Dahir Aweys, said insurgents should continue fighting until all Ethiopian troops had left the country.
Mr Aweys is a founder of the Union of Islamic Courts (UIC) that ruled much of Somalia in 2006, before being ousted by Ethiopian forces backed by Somali government troops.
Al-Shabab is the UIC's youth wing, whose fighters have been behind much of the violence against the Ethiopian and Somali troops.
Aimed at ending years of conflict, this month's deal provides for Ethiopian troops to leave Somalia within 120 days.
It was signed by another top Islamist leader, Sheikh Sharif Sheikh Ahmed, and Prime Minister Nur Adde.
Somalia has experienced almost constant civil conflict since the collapse of Mohamed Siad Barre's regime in January 1991.
The Mogadishu-based Elman Peace and Human Rights Organisation said this week that more than 8,000 civilians had died in the conflict since early last year.
The government says that figure is exaggerated.
The UN has warned that nearly half of Somalia's population is likely to require aid later this year.