BRUSSELS (Reuters) - Belgium will ask Ukraine for clarification on reports that rifles made in Belgium had been used by pro-Ukrainian forces to fight Russian troops inside Russia's western border, Belgian Prime Minister Alexander De Croo said on Monday.

The Washington Post on Saturday reported that anti-Kremlin fighters who launched a cross-border attack from Ukraine into the Russian Belgorod region last month used tactical vehicles originally given to Ukraine by the United States and Poland and carried rifles made in Belgium and the Czech Republic.

"Our defence ministry and its intelligence agencies have started an investigation and are asking for information to determine what has happened exactly," De Croo said on Belgium's Radio 1.

"European weapons are delivered to Ukraine under the condition that they are used on Ukrainian territory with the purpose of defending that territory. And we have strict controls in place to see that this is the case," he said.

De Croo declined to comment on possible consequences if the reports were confirmed.

"We must not get ahead of ourselves here," the prime minister said. "But we are analysing the situation and we would take this very seriously."

(Reporting by Bart Meijer; editing by Philippa Fletcher)