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By Maria Stromova and Saphora Smith

Russian President Vladimir Putin and Iranian President Hassan Rouhani have spoken on the phone and agreed that recent U.S. airstrikes in Syria were unacceptable and violated international law, the Kremlin said Sunday.

The two leaders also called for an "objective, unbiased investigation" into the incident involving chemical weapons which killed more than 100 people last week in Idlib province, Syria, the Kremlin said in a statement on its website.

Putin and Rouhani said they were ready to deepen cooperation to fight terrorism and called for a diplomatic resolution to the war in Syria, according to Russian authorities.

It came as Iranian news wires reported that Rouhani had also spoken to Syrian President Bashar al-Assad on Sunday.

The U.S. fired 59 Tomahawk cruise missiles at Syria overnight Thursday in response to what it believes was a chemical weapons attack earlier in the week, whose victims included at least 25 children.

At least six people were killed in the U.S. airstrikes, Syria claimed, but the Pentagon said civilians were not targeted and the missiles were aimed at a Syrian military airfield in Homs.

Russian President Vladimir Putin shakes hands with Iranian President Hassan Rouhani at the Kremlin in March.ALEXEY NIKOLSKY/SPUTNIK/KREM / EPA

The strike completed a policy reversal for President Donald Trump — who once warned America to stay out of the conflict — and drew angry responses from Damascus and its main ally, Russia.

Earlier on Sunday North Korea vowed to boost its defenses following the U.S. strikes in Syria. A foreign ministry official told the country's state-run news agency that the airstrikes proved that it needed to protect itself against Washington.

"The recent U.S. military attack on Syria is an action of warning," the report said, according to the Associated Press. "We will bolster up in every way our capability for self-defense to cope with the U.S. evermore reckless moves for a war and defend ourselves with our own force."

On Saturday British Foreign Secretary Boris Johnson announced he had canceled a planned trip to Russia because of fast-changing developments in Syria.

Amid rising tensions between Moscow and Washington, Johnson said Saturday the situation in war-torn Syria had "fundamentally" changed following a chemical weapons attack on civilians and the U.S. airstrikes targeting a military airfield.

Responding to Johnson's announcement, Russian Foreign Ministry representative Maria Zakharova described his reasons as "absurd," according to Russian news agency Interfax.

Associated Press contributed.