Chris Coons is concerned at reports that the president wants to reconsider the U.S. relationship with the alliance.
The alliance's secretary general Jens Stoltenberg said that the use of such a poison was "horrific."
Jens Stoltenberg made the comments as he launched the NATO 2030 initiative. He said non-alliance countries like Japan and New Zealand would need to co-operate to balance the rise of China.
Trump spent his two days in London lashing out at allies and creating new rifts, but host Boris Johnson avoided the worst of the flak by limiting his interaction with the president.
Trump cancelled a press conference and left the meeting earlier amid a spat with Justine Trudeau, but Jens Stoltenberg said unity within the alliance was "iron-clad."
Trump told reporters he had done enough press conferences and that he would "probably" leave the London meeting early.
Jens Stoltenberg said he remains a believer in a dual approach of deterrence and dialogue towards Russia.
Turkey is facing widespread condemnation for its military operation against Kurdish-led forces in northeastern Syria.
"Russia has a last opportunity to take the responsible path," NATO's secretary-general warned.
"The launch aimed to confirm the advanced missile system's capabilities and flight characteristics," the Russian Defense Ministry said.
"For over five years, the United States has pursued diplomatic and technical avenues with Russia aimed at preserving the INF Treaty," NATO Secretary-General Jens Stoltenberg said.
All of NATO's 29 members will take part in the exercises, as will the alliance's partners Sweden and Finland.
The retired four-star general shifted in his seat and looked away as Trump branded Germany a "captive" of Russia, but the White House says he was simply underwhelmed by the catering.
Trump has said the U.S. should spend less but he spent more on Europe's defense, all for a Russian threat he disputes is there.
Trump boasted that the NATO leadership is very fond of him, but says Russia must be thinking "if we did like this guy, we made a big mistake."
With the establishment of a new Cyber Operations Center, NATO has launched a new, historic campaign of electronic warfare.
The U.S secretary of state said, "There is broad consensus among all the NATO members that there is no normalization of dialogue with Russia today."
The NATO chief said Kim Jong Un's potentially nuclear-capable missiles threaten Europe, but North Korea said it's the U.S. that's at risk.
"It is more unpredictable, and it's more difficult," Stoltenberg said when asked about the changes in the world over his 30-year career.
Jens Stoltenberg announced 3.8 percent of spike in real terms across Europe and Canada.
Jens Stoltenberg spoke as 4,000 troops prepare to shore up defenses around the Baltic Sea.
Historically, NATO and the EU don't normally work together, but this is changing.
"This is Russia destabilizing Eastern Ukraine," said Jens Stoltenberg.
A 2013 drill on the edge of the Stockholm archipelago was a simulated attack, Stoltenberg said.
Kremlin spokesman Dmitry Peskov said that Moscow would move to protect its interests
Jens Stoltenberg's visit was described as a "milestone" by Ukrainian officials.