Army officials claimed on Tuesday that as much of 75% of the city had been recaptured, including the city centre and government headquarters.

Iraqi forces were rejoined in the battle by Shia militias, who said last week they would boycott fighting while the US was carrying out air strikes.

IS militants seized the strategically important city last summer.

Iraqi Prime Minister Haider al-Abadi said on Twitter that Tikrit, which is Saddam Hussein's hometown, had been "liberated" from IS occupation, but reports said fighting for large parts of the city was ongoing.

Abdul-Wahab al-Saadi, an Iraqi army commander Tikrit's Salahuddin province, said his forces fighting in from the west were still 300m (1,000ft) from the city centre.

There are about 3,000 Iraqi soldiers and police fighting to liberate the Sunni city, with support from about 20,000 Iran-backed Shia militias, known collectively as the Popular Mobilisation units, as well as local tribesman and residents.

Some of the militias appear to have reversed an earlier decision to freeze their participation in the offensive while the US-led coalition carried out strikes.

Tuesday's advances are the most significant in the government offensive in the city, which began on 2 March but was stalled as it waited for air support and ground reinforcements.

Ammar Hikmat, deputy governor of Salahuddin province, where Tikrit lies, said pro-government forces were "pushing forward toward the presidential complex and have already entered parts of it".

"I think the whole city will be retaken within the coming 24 hours," he told the Associated Press news agency.

Recapturing Tikrit is seen as strategically vital in the battle for Mosul, Iraq's second largest city. Mosul, which is north of Tikrit along the Tigris river, was captured by IS in June last year during a lightning advance across the country.