London — Iran said it sank an Iraqi gunboat in a sea battle near the northern end of the Gulf. The Iranian news agency IRNA, monitored here, said the Iraqi vessel tried to attack Iran's Nowruz oil field. Previously shattered wells in the Nowruz field have been pouring oil into the Gulf waters for more than two months. The resulting oil slick in the Gulf has become something of a weapon. Analysts say the slick may be more to Iraq's advantage in the war, since it threatens to clog the engines of tankers calling at Iran's oil terminals and may keep them away. Iraqi terminals on the Gulf are not in use.