Polish security forces fired water cannon at rock-throwing migrants on the border with Belarus on Tuesday, and NATO reiterated its support for Warsaw in a crisis that has left thousands stranded on the frontier in icy temperatures.

Video footage released by Polish authorities showed migrants also throwing bottles and logs across a barbed-wire border fence, and using sticks to try to break through.

Seven police were hurt in the violence, the latest in a crisis the European Union says is orchestrated by Belarus - an ally of Russia - in retaliation for EU sanctions imposed over a crackdown on political protests, a charge that Minsk denies.

Up to 4,000 migrants, mostly from Iraq and Afghanistan, are now waiting in freezing forests on what is not only Poland's frontier but is also the external border of the EU and NATO, the Western military alliance.

"We are deeply concerned about the way the (Belarusian leader Alexander) Lukashenko regime is using vulnerable migrants as a hybrid tactic against other countries," NATO Secretary General Jens Stoltenberg told alliance defence ministers meeting in Brussels. "We stand in solidarity with Poland and all the allies affected."

Lithuania and Latvia, which like Poland are members of NATO and the EU, have also reported a sharp increase in attempts to cross from Belarus since summer.

At least eight migrants have died at the border during the crisis. One, a 19-year-old Syrian man, was buried on Tuesday in the northeastern Polish village of Bohoniki. read more

A nine-year old Kurdish boy who has had both legs amputated was among those stuck between the lakes, swamps and forests at the frontier after Poland refused to let them in and Belarusian forces prevented them heading back. read more

'ENORMOUS SUFFERING'

"We can see enormous suffering of people who are left in limbo," said Dunja Mijatovic, human rights commissioner for the Council of Europe, a European rights watchdog that is larger than the EU and also counts Russia among its members.

After visiting a migrants' aid centre in a Polish town nearby the border, she said: "We need to find a way to de-escalate, to make sure that the focus is on stopping the suffering."

Relations between Belarus and the EU worsened after a contested presidential election last year in which Lukashenko, who has held power since 1994, claimed victory. That triggered mass street protests and, in turn, a police crackdown.

The EU agreed on Monday to impose more sanctions on Belarus to target airlines, travel agencies and individuals involved in pushing migrants towards the border.

The EU and NATO have asked Russia, Lukashenko's most important ally, to make him end the crisis. The West has also warned the Kremlin over what NATO says is a Russian military buildup on the border with neighbouring Ukraine.

In Brussels, French Armed Forces Minister Florence Parly said Europe was keeping a close eye on both the Belarus-Polish border and Russia's activity near Ukraine.

"It is an unsupportable instrumentalisation (of migrants)," she said.

Italian Defence Minister Lorenzo Guerini said the West was acting together to "firmly condemn the Belarusian regime."

Russian President Vladimir Putin and Lukashenko discussed the matter on Tuesday, Russian state news agency TASS quoted the Kremlin as saying.

Belarusian state news agency BELTA said border guards had started moving migrants who gathered at a closed crossing point to a reception centre further away from the frontier.

Moscow has dismissed a U.S. State Department comment that the crisis was meant to distract attention from Ukraine, from which Russia annexed Crimea in 2014. Russia has also backed separatists fighting government troops in eastern Ukraine.

Additional reporting by Pawel Florkiewicz, Marko Djurica, Anita Kobylinska, Fedja Grulovic, Anna Koper, Matthias Williams, Polina Devitt, Writing by Gabriela Baczynska, Editing by Timothy Heritage.