(CNN) New airstrikes targeted a town in Syria that was hit by a chemical attack earlier this week, activists said, less than a day after the US bombarded a Syrian air base to "send a message" to the Assad regime.

It wasn't immediately clear who conducted the strikes on Khan Sheikhoun, which was hit on Friday and Saturday, though only Russian and Syrian regime aircraft have been bombing that area of rebel-held Idlib province.

The latest attacks come after a missile strike early Friday by the United States on a base in western Syria that the United States says was used to launch Tuesday's chemical attack , which left more than 85 people dead and hundreds more injured.

The new strikes came as Russia, the Syrian regime's main ally, sent a frigate armed with cruise missiles to a port in western Syria in an apparent show of force in response to the US action.

Key developments

The Syrian air force resumed flight operations at the base the US struck Friday.

Pentagon investigating possible Russian involvement in chemical attack that prompted US strike.

US officials say about 20 planes were destroyed in Friday's attack on the Shayrat base.

US ambassador to the UN Nikki Haley tells United Nations the United States is "prepared to do more."

Russia says United States has no proof chemical weapons were used Tuesday.

At least one woman was killed and three other people were injured in Saturday's strikes in Khan Sheikhoun, two activists in the town said. The strike that killed the woman happened in a residential neighborhood, activist Alaa Al-Youssef said.

It wasn't clear from where the strikes were launched, but the Syrian air force resumed flight operations at the base the United States struck Friday, two pro-regime media outlets and an opposition group said Saturday.

A video on Instagram, posted Saturday by a reporter from the state-run Russia-24 outlet, purported to show a jet rolling down a tarmac at the air base. The caption reads: "Return to work at Shayrat."

#shayratairbase возобновила работу через сутки после удара крылатыми ракетами. #СильноБили #syria #usa #Россия24 A post shared by Евгений Поддубный (@evgeny.poddubny) on Apr 8, 2017 at 3:53am PDT

A US defense official said Friday's strikes were not intended to damage runways or fully disable the base. Instead, the strikes hit aircraft, fuel storage, weapons dumps and other equipment, aiming to send a message to the Syrian regime that any use of chemical weapons would not be tolerated, the official said.

In the aftermath of the US strike, Russia pledged to help strengthen Syria's air defenses. Russian state media reported that a frigate, the Admiral Grigorovich, would call at a logistics base at Tartus, Syria. It had earlier picked up supplies at the Black Sea port of Novorossiysk.

The Russian frigate, the Admiral Grigorovich. The ship, which is armed with cruise missiles, was reportedly entering the Mediterranean en route to a logistics site in Syria, Russian state media said.

NATO called it one of the largest deployments from Russia in decades.

Retired US Lt. Gen. Mark Hertling, a CNN military analyst, said the frigate appeared to be a show of force by Russia, which since late 2015 has been conducting airstrikes in Syria against forces opposing the Syrian regime.

"I think the Russians were caught off guard (by the US strikes)," Hertling said. "So they want to make sure they're tracking those (US) ships."

An airstrike killed 15 civilians, including four children, in Syria's northwestern province of Idlib on Saturday, the Syrian Observatory for Human Rights monitor said. The monitoring group said the strike on the village of Urum al-Joz was suspected to have been carried out by Russian planes, which operate in support of the Syrian government. Syria's Civil Defence volunteer group, also known as The White Helmets, said the death toll was 16, with 20 people injured.

A US-led coalition also has long been conducting airstrikes in Syria, against ISIS targets in the country. On Saturday, a suspected coalition strike killed at least 15 civilians in north-central Syria near Raqqa, the de facto capital of ISIS, a local activist group and the state-run SANA news outlet said.

US quiet on next steps

US President Donald Trump said he ordered the Tomahawk missile strike on the Shayrat base because the United States believed aircraft that carried out Tuesday's chemical attack were launched from there.

Trump praised his military for Friday's missile strike. "Congratulations to our great military men and women for representing the United States, and the world, so well in the Syria attack," Trump posted Saturday to Twitter.

Congratulations to our great military men and women for representing the United States, and the world, so well in the Syria attack. — Donald J. Trump (@realDonaldTrump) April 8, 2017

The White House late Friday refused to say whether its strike on Shayrat air base was a one-off action or part of a new strategy designed to hobble the military capabilities of President Bashar al-Assad. Nor would it say whether the United States believed Assad should step down after the chemical attack on Khan Sheikhoun that killed at least 80 people and injured dozens more on Tuesday.

A satellite photo of the Shayrat air base after the US missile strikes.

The White House refused to discuss next steps. Press Secretary Sean Spicer said Trump would not "telegraph his next move." Speaking to reporters at Trump's Mar-a-Lago resort in Florida, Spicer said the US missile attack was "very decisive, justified and proportional."

He declined to say whether Trump now believed Assad should relinquish power. "First and foremost the President believes that the Syrian government [and the] Assad regime should abide by the agreement they made not to use chemical weapons," Spicer said.

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US Secretary of State Rex Tillerson said Russia had failed to honor an agreement to guarantee the elimination of Syria's chemical weapons. "Clearly, Russia has failed in its responsibility to deliver on that commitment from 2013," he said. "So either Russia has been complicit or Russia has been simply incompetent in its ability to deliver on its end of that agreement."

British Foreign Secretary Boris Johnson, meanwhile, canceled his planned Monday visit to Moscow, citing developments in Syria.

In a statement Saturday, Johnson said his priority was to continue contact with the United States and others in the run-up to a meeting of G-7 foreign ministers in Italy on Monday and Tuesday, with the aim of building "coordinated international support for a ceasefire on the ground and an intensified political process."

North Korea responded to the US strike with a comment provided to CNN International Correspondent Will Ripley, who is in Pyongyang.

"In some quarters, they say these military attacks are a warning shot against the DPRK (North Korea), but we are not at all frightened by those actions," North Korea said in a comment provided to Ripley from "relevant government departments."

"The Syrian crisis is a bloody lesson which has been deeply engraved into the bones of the Korean people, the lesson being that we should never harbor any illusions about imperialists," the comment said.

Russia complicit?

US officials have said the Pentagon is looking for any evidence the Russian government knew about or was complicit in Tuesday's chemical attack.

A US military official told CNN the Pentagon was examining specifically whether a Russian warplane had bombed a hospital in Khan Sheikhoun five hours after the initial chemical attack, with the aim of destroying evidence.

Russia flatly denies the allegations.

Samples are taken from the site of a suspected chemical attack in Khan Sheikhoun.

Russian President Vladimir Putin denounced the US strike as an "act of aggression" and said it violated international law. Tillerson said he was disappointed in the Russian response.

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Russian Sen. Alexey Pushkov said the disappointment was mutual and suggested Tillerson's comments were made to gain leverage in upcoming US-Russia talks. Tillerson is due to meet Russian Foreign Minister Sergey Lavrov in Moscow next week.

Russia: No evidence of chemical weapons at air base

Russia has claimed Tuesday's deaths in Khan Sheikhoun were caused by a Syrian regime airstrike on a rebel-controlled chemical weapons facility.

Russia's Defense Ministry said Saturday that Washington has not provided any evidence that chemical weapons came from the Shayrat base. "No one present at the airport are wearing gas masks and everyone feels perfectly normal," ministry spokesman Igor Konashenkov said after the US strike.