BAGHDAD — Iraqi forces face a prolonged fight to reclaim the northern city of Tikrit from the Islamic State, military experts and analysts said, as pro-government forces continued a large-scale offensive to oust the militant group from one of its major urban strongholds. The militants on Tuesday carried out suicide attacks and detonated roadside bombs on the outskirts of the city, about 110 miles northwest of Baghdad, to slow the Iraqi troops’ advance.