HARARE, Nov 14 (Reuters) - Four tanks were seen heading towards the Zimbabwe capital Harare on Tuesday, witnesses said, a day after the head of the armed forces said he was prepared to "step in" to end a purge of supporters of ousted vice-president Emmerson Mnangagwa.

A Reuters witness saw two other tanks parked beside the main road from Harare to Chinhoyi, about 20 km (14 miles) from the city. One of the tanks, which was pointed in the direction of the capital, had come off its tracks. Soldiers on the scene refused to talk to Reuters.

(Reporting by MacDonald Dzirutwe; Writing Ed Cropley; Editing by James Macharia)