Kiev (AFP) - Ukraine announced Thursday that anyone crossing in or out of areas controlled by pro-Russian separatists in the east of the country will have to show a passport.

The rule, announced by the border guards service, was aimed at sealing off the separatist region in what will de facto become an internal border in the seven-month conflict.

Ukrainian citizens will be allowed to travel in and out of the area "on condition of showing passports," the border guards service said in a statement.

Foreigners will "be sent to filtration points to determine the purpose of their visit" and will have "to show a passport or the required visa," the statement said.

Officials said the passport regime will apply to the area of what the government calls the "anti-terrorist operation," but did not specify the exact outlines of the area. The rule appeared to signal the creation of an increasingly formal border between the Russian-backed separatist zone and the rest of Ukraine.