Updated at 12:15 p.m. EST November 29, 2018

President Donald Trump has abruptly canceled a face-to-face with Russian President Vladimir Putin at the imminent G20 summit in Argentina, blaming Moscow’s seizure of Ukrainian naval vessels last weekend.

The cancellation may have more to do with former Trump lawyer Michael Cohen pleading guilty to lying to Congress over the Russia investigation, which would complicate a meeting with Putin even at less perilous moments, but it’s still a high-profile snub.

Ukraine would like Trump to back up with the cancellation with strong gestures — more lethal military assistance, plus more sanctions — Ukraine’s Ambassador to the U.S. Valeriy Chaly told The Daily Beast ahead of Trump’s meeting cancelation on Wednesday.

“Above all, we need a strong and clear message to the aggressor—this blatant act of aggression will not be tolerated,” he said, just as the world’s top economic powers prepared to gather in Buenos Aires on Friday.

“Since Russia only understands the position of strength—we need to prevent further escalation by tough measures—and targeted sectoral sanctions could be one of them.”

The request for more lethal military assistance is likely to be well received on Capitol Hill, which has already passed a hefty budget of $250 million for military aid to Ukraine in 2019, assistance that was championed by the U.S. special representative to Ukraine, Kurt Volker.

National Security Adviser John Bolton has continued the trend, backing increased sanctions for Russian misbehavior and even publicly entertaining the idea of further arming Kiev, a move that could rattle Moscow.

But it may take a while for Trump to catch up with his team’s animus toward Putin in what is shaping up to be a testy face-to-face after the near-bromance the two leaders displayed in their Helsinki meeting.

Trump left it to his outgoing U.N. Ambassador Nikki Haley to blast Russia on Monday for “another reckless Russian escalation,” and Volker took to Twitter to condemn it, tweeting Monday, “Russia rams Ukrainian vessel peacefully traveling toward a Ukrainian port. Russia seizes ships and crew and then accuses Ukraine of provocation???”

“It was obviously a flagrant violation of international law, it was, I think, a cavalier use of force,” Defense Secretary Jim Mattis said Wednesday. “When you think that there is a treaty between the two countries that prevents exactly what happened, it just shows that Russia cannot be counted on right now to keep its word.”

Trump’s criticism was far more tepid. He told The Washington Post he might skip the Putin meeting. “I don’t like that aggression. I don’t want that aggression at all,” he said.

That may have been to preserve some room to maneuver ahead of the G-20 summit that is shaping up to be one uncomfortable confrontation after another for the U.S. president.

Trump seems to relish his ongoing trade smackdown with China, but he is studiously not planning to meet the Saudi crown prince, after giving Riyadh a pass for the murder of a Washington Post journalist, citing hoped-for weapons sales to the kingdom.

And now he has to confront a recalcitrant Putin over Ukraine, when he still can’t seem to muster as much anger over Russia’s actions as he does over U.S. media coverage of his administration or NATO members’ failure to invest as much as he thinks they should as fast as they should in defense.

Putin goes into the meeting defiant, claiming the Russian coast guard was provoked into firing on and seizing three Ukrainian naval boats and several sailors over the weekend.

The Kerch Strait connects the Black Sea with the Sea of Azov, which is bordered by Russia, Ukraine, and the Crimean Peninsula, which Russia seized in 2014. Despite a 2003 agreement with Ukraine that they both have rights to the Sea of Azov, Russia has taken the view that the Kerch Strait is part of Crimea, which it insists is part of Russia—a seizure the rest of the world considers illegal. A $4 billion bridge built by Russia after the annexation of Crimea now spans the strait.

Moscow and Kiev have each accused the other of harassing shipping traffic in the area, friction exacerbated by Russia's continued support of an insurgency in eastern Ukraine. In March, Ukraine detained a fishing vessel sailing from Crimea, which Russia used as a reason to increase military patrols, according to the AP. It also greatly increased “inspections” of Ukrainian vessels, slowly choking off two major Ukrainian ports on the Sea of Azov. Ukrainian officials say they are now effectively under blockade.

Ambassador Chaly says if the latest move by Moscow isn’t met with a harsh response, Russia will just keep pushing.

“This is a classic Russian M.O.—they will bite here and there and see what is the response,” he said. “If it is too weak—next time they will grab more.”

Ukraine’s President Petro Poroshenko warned this week that “full-scale war” is threatened by Russia’s actions, while Putin suggested the crisis is all the result of Ukrainian provocations and political maneuvering by Poroshenko in advance of March elections.

Ukraine is not a NATO member, but has expressed “full support” for the country, and Poroshenko would like to see stepped-up patrols in the Black Sea. “We cannot accept this aggressive policy of Russia,” Poroshenko told Germany’s Bild newspaper. “First it was Crimea, then eastern Ukraine, now he wants the Sea of Azov. Germany, too, has to ask itself: What will Putin do next if we do not stop him?”

Among Chaly’s requests is one for U.S. assistance securing the return of Ukrainian sailors seized in the incident, and a new wish list for lethal aid: more naval cutters like the two the U.S. has already gifted Ukraine, plus maritime defensive lethal equipment, such as anti-ship missiles, enhanced radar, and maritime surveillance capabilities.

He said his government had relayed the request to the State Department, but that conversations are at an early stage.

Chaly also said his country wants NATO to step up its presence in the area. “ NATO military capabilities would contribute much in monitoring and understanding the broader scale of the security situation in the region,” he said.

Chaly defended his government’s decision to declare martial law in his country, saying it won’t delay the March presidential election but calling it necessary to shift more of the country’s resources to defense “with an eye to possible Russian wide-scale offensive as prompted by the continuing Russian massive military build-up along the whole stretch of its border with Ukraine.”

The Russian embassy in Washington did not immediately respond to request for comment on Chaly’s request for additional military assistance.

“We continue to call on Russia to deescalate the situation and release the illegally detained crew and vessels,” said a senior administration official Wednesday, speaking anonymously because the official was not authorised to speak publicly. The official would not comment on the Ukrainian request .

“The Department of Defense supports U.S. government efforts to find a resolution to this situation, but will not speculate on possible military responses,” emailed Pentagon spokesman Eric Pahon Wednesday. He added, “Russia's aggression is not simply a matter for Ukraine. It is a threat to the region, to Europe, to the United States, and to the stability of the international order.”

“Regarding future military assistance to Ukraine, we will continue to evaluate the specific capability needs of Ukraine’s forces in collaboration with the Government of Ukraine and our partners,” a State Department spokesman emailed, insisting on anonymity.

NATO spokeswoman Oana Lungescu emailed that three NATO members, Bulgaria, Romania, and Turkey, already have capabilities deployed in the Black Sea region. “So there is already a lot of NATO in the Black Sea, and we will continue to assess our presence in the region,” she said.

Another senior administration official offered a simple if snarky solution for Kiev to get unconditional support from Trump: “Gee, if only Ukraine would buy a bunch of weapons from the United States, then it would all be okay,” the official said, speaking anonymously while alluding to Trump’s recent backing of Saudi Arabia because of weapons sales, despite the murder of Washington Post journalist Jamal Khashoggi.

Editor’s note: This story has been updated with news that Trump canceled his meeting with Putin, plus comment from the State Department and Pentagon.