* Islamic State hemmed into centre of Sirte

* But recent suicide attacks show it can still strike back

* Retaking city would be step forward for unity government

TRIPOLI, June 16 (Reuters) - A suicide car bomber hit a Libyan police station west of the besieged Islamic State stronghold of Sirte on Thursday, killing 10 people and wounding seven, officials said.

The attack at Abu Grain, on the coast road some 140 km (85 miles) west of Sirte and 100 km south of Misrata, showed the militants, although on the back foot in the battle for Sirte, can still strike outside the city.

Brigades from Misrata are leading the campaign to recapture Sirte and have engaged in fire fights with Islamic State since pushing to the edge of the city centre last week.

Rida Issa, a spokesman for the brigades, said Thursday's casualties were believed to include civilians as well as policemen. Earlier on Thursday the brigades' media office said two other car bombs had been destroyed in Sirte before they could be used.

The brigades, which are aligned with Libya's U.N.-backed unity government, say they have Islamic State pinned back to an area about 5 km by 5 km in Sirte's city centre.

A bombing that hit a field hospital about 50 km behind the front line on Sunday also showed the brigades are vulnerable to attacks by militants who have either escaped from Sirte or are based outside the city.

The campaign against Sirte was launched last month after Islamic State advanced up the coastal road towards Misrata, briefly occupying Abu Grain and several nearby villages.

Islamic State expanded into Libya in late 2014, taking full control of Sirte the following year and making it its most important base outside the Middle East. It has established cells and carried out attacks in several Libyan towns and cities but has struggled to retain territory in the North African state.

The U.N.-backed Government of National Accord (GNA) is seeking to unite disparate political and armed factions to take on Islamic State and end the turmoil that has plagued Libya since Muammar Gaddafi was toppled in an uprising five years ago. (Reporting by Ahmed Elumami; Writing by Aidan Lewis; Editing by Robin Pomeroy)

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