HELSINKI (Reuters) - Finland will propose a law next week allowing border agents to block asylum seekers trying to enter from Russia, the prime minister said on Sunday, a decision that could cause Helsinki to temporarily breach its international commitments.

Finland shut its border with Russia last year to stop a growing number of arrivals from countries including Syria and Somalia, and accused Moscow of weaponising migration against it and the European Union, an assertion the Kremlin denies.

Finland annoyed Russia last year by abandoning its long-held military non-alignment and joining the NATO alliance in response to Moscow's invasion of Ukraine. It has also signed a bilateral defence pact with the United States.

The draft law proposed by the right-wing governing coalition acknowledges that turning migrants back to Russia without processing their asylum applications would be in breach of Finland's international human rights commitments, but says its use will be temporary and limited.

Prime Minister Petteri Orpo said the proposal would go to parliament next week, where it will be submitted to the constitutional committee for review.

It will need five-sixths of votes cast in parliament to pass - the high bar required for constitutional matters - and success is not certain.

"As this phenomenon is in Russia's hands - who comes, where from and when, to Finland's border - we cannot permit it. Therefore we have to augment our legislation," Orpo told reporters.    

Fewer than 40 migrants have crossed the border from Russia through the wilderness this year since Finland shut its land crossings, while some 1,300 arrived through the crossings last year, Border Guard data showed. One railway crossing remains open, for cargo only.

Finnish leaders say the legislation is still needed because migrants could start arriving again as warmer weather makes travel easier.

"There are still thousands of people in Russia ... who are waiting for an opportunity to reach the Finnish border," Orpo said, citing intelligence reports.

Finland shares a 1,340 km (830 mile) border with Russia.    

The bill would allow border authorities to turn back asylum seekers who cross from Russia, with or without using force. But it would not apply to children and disabled people, whose asylum applications Finland would continue to accept.

Contacted by Reuters on May 3, Finland's largest opposition group, the Social Democrats, said they would not commit to supporting the bill before parliament had discussed the final version. 

Finland's ombudsman for non-discrimination, Kristina Stenman, said it would be alarming if Finland refused to accept asylum applications from people arriving at its borders.

(Reporting by Anne Kauranen and Essi Lehto in Helsinki, Terje Solsvik in Oslo; editing by William Maclean and Kevin Liffey)

By Anne Kauranen and Essi Lehto