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Warplanes on Saturday struck the Syrian town ravaged by a deadly chemical attack earlier this week, activists said, as Russia's foreign minister defended the Syrian government in a phone call with Secretary of State Rex Tillerson.

The fresh airstrikes were on the rebel-held town of Khan Sheikhoun, where about 100 civilians were killed in Tuesday's chemical attack, according to the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees, an activist collective.

NBC News could not immediately confirm the strikes and any potential casualties.

The chemical attack prompted the U.S. to rain down 59 Tomahawk cruise missiles on Syria's Shayrat Airfield early Friday, marking the first time Washington has directly targeted Syrian government forces since the civil war began in 2011.

In a letter Saturday to Congress, Trump officially explained his rationale for getting America involved more forcefully in Syria.

"I directed this action in order to degrade the Syrian military's ability to conduct further chemical weapons attacks and to dissuade the Syrian regime from using or proliferating chemical weapons, thereby promoting the stability of the region and averting a worsening of the region's current humanitarian catastrophe," he wrote to House Speaker Paul Ryan and Senate President Pro Tempore Orrin Hatch.

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Related: Was the U.S. Missile Strike on Syria Legal?

"The United States will take additional action, as necessary and appropriate, to further its important national interests," Trump continued, adding that his report is "part of my efforts to keep the Congress fully informed."

Trump also suggested on Twitter that the U.S. wasn't specifically trying to hit runways at the airfield during the operation.

The reason you don't generally hit runways is that they are easy and inexpensive to quickly fix (fill in and top)! — Donald J. Trump (@realDonaldTrump) April 8, 2017

Trump's decision was welcomed by the Syrian rebels and their main backers, including Turkey and Saudi Arabia, but harshly condemned by Russia and Iran, who support Syrian President Bashar al-Assad and said striking his forces would complicate the struggle against extremist groups.

The office of the Russian Foreign Ministry said in a Facebook post that the Americans initiated a phone call with Russia on Saturday, ahead of Tillerson's planned trip to Moscow next week.

Russian Foreign Minister Sergy Lavrov warned Tillerson that attacking Syria would hinder the fight against terrorism and only threaten global security around the region. He also said the Syrian government did not engage in chemical warfare earlier this week, according to the Russian Foreign Ministry — despite what activists and the U.S. government have said.

Russian military officials also said Saturday that if the purpose of the U.S. bombing the Shayrat Airfield was to take out potential chemical weapons, none were found at the site following the strike.

Related: Trump's Syria Airstrike Sends a 'Critical Message': Ex-Pentagon Official

"Once again this resembles the story with Colin Powell's white powder or reports to the U.K. prime minister about the claimed chemical weapons in Iraq," said a spokesman for Russian Defense Ministry Maj. Gen. Igor Konashenkov, according to the official state news service TASS.

The mounting tensions between Moscow and Washington have put pressure on their respective allies to take sides.

British Foreign Secretary Boris Johnson said Saturday he was canceling a planned trip to Russia because of fast-moving events in Syria.

He condemned Russia's continued defense of Assad "even after the chemical weapons attack on innocent civilians."

His decision not to visit Moscow on Monday prompted a Russian Foreign Ministry spokesperson to describe his reasoning as "absurd" and the Russian Embassy in Britain to send out a mocking tweet that featured Tchaikovsky's 1812 Overture — written to commemorate Napoleon's disastrous retreat from Russia.

.@BorisJohnson cancelled his visit to Moscow: theatrics for lack of argument? Safer in G7 pack? Listen to our soft power response: pic.twitter.com/PjSa5psT6p — Russian Embassy, UK (@RussianEmbassy) April 8, 2017

Tillerson meanwhile plans to meet with G-7 foreign ministers in Europe next week before going on to Moscow. Johnson said Tillerson will be able to give a "clear and coordinated message to the Russians."