Washington (CNN) Under sustained pressure from Congress, President Donald Trump has imposed long-overdue, legally mandated sanctions on Russia for its poisoning of an ex-spy in the United Kingdom .

Russian agents were accused of using a banned nerve agent to carry out a failed March 2018 attack on British soil against former Russian spy Sergei Skripal and his daughter, Yulia. The Kremlin has denied the allegations.

The attack left the Skripals in a coma and killed another woman.

Trump signed the executive order on sanctions Thursday, more than six months after they were due and a day after a call with President Vladimir Putin.

'A good talk'

The White House said the call was focused on Trump's offer to help Russia with "vast wildfires afflicting Siberia" and trade between the two countries. "We had a good talk, a short talk, but a good talk," Trump told reporters Thursday.

Moscow, which issued a statement about the call hours before the White House did, said Trump had initiated it. Neither the American nor the Russian statement mentioned any discussion of the sanctions, but Putin praised Trump's offer of firefighting help "as a guarantee that in the future it will be possible to restore full-fledged relations between the two countries."

The Russian statement went on to say the two leaders had "agreed to continue contacts in a telephone format, as well as in face-to-face meetings."

Trump has been reluctant to impose sanctions on Russia amid his efforts to improve relations between Washington and Moscow, and as recently as this week he continued his attempts to cast doubt on Russian interference in the 2016 presidential election. Last year, the Trump administration was slow to enact sanctions punishing Russia for that interference.

A Senate aide said it was widely known among Democrats and Republicans working on sanctions that Trump was angry when the Treasury and State departments made the determination last fall that Russia was failing to prove it was no longer using chemical weapons, triggering a second round of sanctions under US law.

Though Treasury and the State Department had done all the legwork to prepare the mandatory sanctions, the White House failed to act on them.

With the sanctions more than six months overdue, lawmakers from both sides of the aisle moved this week to apply pressure.

The House Foreign Affairs Committee's top Democrat and Republican sent a letter to Trump on Monday demanding that the administration meet its congressionally mandated obligations.

'Unacceptable'

The Chemical and Biological Weapons Control and Warfare Elimination Act of 1991 "mandated the second round of sanctions to be imposed within three months, yet well over a year has passed since the attack," wrote Reps. Eliot Engel, a New York Democrat, and Michael McCaul, a Texas Republican. "Therefore, we urge you to take immediate action to hold Russia fully accountable for its blatant use of a chemical weapon in Europe."

They added that a failure to do so "is unacceptable and would necessitate that Congress take corrective action."

Agathe Demarais, global forecasting director at The Economist Intelligence Unit, said the fact of the sanctions wasn't a surprise, given wide bipartisan backing and the Mueller report's conclusive finding that Russia did interfere in the 2016 election -- but the timing was.

"The implementation of the CBW Act sanctions is the responsibility of the executive branch, giving Mr. Trump ultimate authority over them; had he wished to, he could have moved much earlier, but he didn't, and didn't give any warning signs he was about to impose these sanctions," Demarais said.

The administration appears to be downplaying the sanctions. White House spokesman Hogan Gidley said Friday that Trump had spoken with new UK Prime Minister Boris Johnson on Thursday, but made no mention of the Russia sanctions.

Instead, Gidley said they discussed areas of further cooperation, especially relating to trade, 5G network technology and global security.

When asked, a senior administration official told CNN that "after the first round of sanctions in response to Russia's use of Novichok in an assassination attempt against a private citizen in the United Kingdom, Russia did not provide the assurances required under US law, so we are imposing the second round of sanctions."

Russia was then expected within 90 days to certify to the US that it was no longer using chemical weapons and to allow inspectors to prove it -- or face a more stringent set of sanctions, under the law.

But after the State Department announced in early November 2018 that Russia had failed to meet these terms, no new penalties were imposed.

Rather, State Department spokeswoman Heather Nauert said at the time that they were "consulting with Congress regarding next steps as required 90 days after the initial determination on August 6, 2018." In February, CNN reported that the State Department had not been consulting with Congress as promised.