WASHINGTON — President Trump said on Wednesday that Russia likely knew of the Syrian government’s plan to gas its own people in advance of a chemical weapons attack last week in northwestern Syria, asserting that United States relations with Moscow were at an “all-time low.”
Asked whether it was possible that Syrian forces could have launched the chemical attack without Russia’s knowledge, Mr. Trump said: “It’s certainly possible; I think it’s probably unlikely.”
“I would like to think that they didn’t know, but certainly they could have. They were there,” Mr. Trump said of the Russians during a news conference at the White House with Jens Stoltenberg, the secretary general of NATO.
The remarks amounted to an explosive suggestion at a time of heightened tension between the United States and Russia after the chemical attack, which was followed by American missile strikes on the airfield in Syria from which it was launched.
Even as they have intensified their criticism of Russia for backing President Bashar al-Assad of Syria, other senior Trump administration officials, including Jim Mattis, the secretary of defense, and Rex W. Tillerson, the secretary of state, have been careful to say there is no evidence proving that Moscow had foreknowledge the Assad regime planned to launch a sarin gas assault.
“Right now, we’re not getting along with Russia at all — we may be at an all-time low in terms of relationship with Russia,” Mr. Trump said on Wednesday. “This has built for a long period of time, but we’re going to see what happens.”
The remarks were the latest evidence of Mr. Trump’s turnabout on President Vladimir V. Putin of Russia, a leader he praised during the presidential campaign but has now moved to isolate since the Syrian chemical weapons attack.
Mr. Trump also made a stark reversal on NATO, which he derided as obsolete during his bid for the White House, but said Wednesday had transformed into an effective alliance since he took office.
“I said it was obsolete; it’s no longer obsolete,” Mr. Trump said during the 30-minute appearance with Mr. Stoltenberg. He called NATO a “great alliance” and “the bulwark of international peace and security.”
Mr. Trump attributed his change of heart to unspecified transformations within NATO that he said were a direct response to criticism he had leveled that the alliance was not doing enough to combat terrorism.
“I complained a long time about that,” Mr. Trump said, “and they changed.”
It was not clear what the president was referring to; NATO forces have been fighting alongside the United States in Afghanistan for more than a decade, an effort focused on combating terrorist groups including the Taliban.
His comments came hours after a senior White House official said the Trump administration had supported the admission of Montenegro into NATO in part to counter the influence of Russia in the small Balkan nation. Speaking on the condition of anonymity, the official cited “credible reports” that Moscow backed a plot for a violent Election Day attack there last fall.
Mr. Trump on Tuesday signed the paperwork allowing Montenegro to enter NATO, two weeks after the Senate approved the move in a March 28 vote. Its admission, White House officials said in a statement, should signal to other nations aspiring to join the alliance that “the door to membership in the Euro-Atlantic community of nations remains open and that countries in the Western Balkans are free to choose their own future and select their own partners without outside interference or intimidation.”
But the official said Russia’s meddling in Montenegro and Ukraine, as well as its actions in Syria, would be a strong theme in Mr. Trump’s NATO discussions, most likely highlighting the president’s commitment to a mutual defense alliance that he questioned before taking office. During the meetings, the administration said, Mr. Trump and Mr. Stoltenberg will talk about the need for Russia to cease its interference in eastern Ukraine and support peace efforts there.
The new dynamic is a substantial shift from Mr. Trump’s stance during his presidential campaign, when he often questioned the usefulness of NATO and regularly praised Mr. Putin.
On Wednesday, White House officials said, the president planned to express an ironclad commitment to the alliance even as he explored ways to pressure member nations to pour more money into their military budgets to help shoulder the burden of securing the North Atlantic.
Mr. Trump has often argued that NATO members cheat the United States by refusing to spend enough to defend themselves. Last month, after a visit by the German chancellor, Angela Merkel, he said that her country owed “vast sums” to the alliance and should pay the United States more for protection.
The Trump administration wants all NATO allies to deliver a plan by the end of the year on how they will meet a pledge agreed to in 2014 that member nations would move toward devoting 2 percent of their gross domestic products to their military budgets by 2024, with at least 20 percent to be spent on equipment procurement and research and development.
Mr. Trump and Mr. Stoltenberg were also to discuss NATO’s role in supporting the fight against the Islamic State and other terrorist groups, as well as military operations in Afghanistan and Iraq, White House officials said.