Russia’s seizure of three Ukrainian ships has served up a new test for President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE.

The issue is looming over the Group of 20 (G-20) summit this weekend in Buenos Aires, Argentina, where Trump will be under pressure to deliver a firm response to Moscow.

Trump on Thursday canceled a one-on-one meeting with Russian President Vladimir Putin because “ships and sailors have not been returned to Ukraine from Russia.”

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The incident off the coast of Ukraine’s Crimean Peninsula further complicates Trump’s effort to repair relations with Moscow at a time of near peak tensions, following Russia’s effort to meddle in the 2016 presidential election.

Ukraine accused Russia on Sunday of ramming one of its boats and opening fire on and capturing three vessels and 24 crewmembers off the coast of Crimea, which Russia annexed in 2014 to international condemnation. Russia’s federal security service, the FSB, said the boats were operating unlawfully in its territorial waters and Moscow has since jailed the sailors. Ukraine has also accused Moscow of a de facto blockade on two of its major ports in the Azov Sea.

U.S. Ambassador to the United Nations Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE and Secretary of State Mike Pompeo Michael (Mike) Richard PompeoPutin nominated for Nobel Peace Prize The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Pompeo accused of stumping for Trump ahead of election MORE both decried Russia’s actions as a violation of international law. Trump himself has remained relatively quiet on the issue, telling The Washington Post in an interview Tuesday that he didn’t like “that aggression” and suggesting he could cancel the Putin meeting before pivoting to a discussion about insufficient spending by NATO partners.

Trump’s decision to cancel the meeting is welcome news to those who argued it would send the wrong message given Moscow’s latest behavior. Still, some are demanding that Trump take further steps.

Sen. Bob Menendez Robert (Bob) MenendezKasie Hunt to host lead-in show for MSNBC's 'Morning Joe' Senators ask for removal of tariffs on EU food, wine, spirits: report VOA visa decision could hobble Venezuela coverage MORE (D-N.J.) has called on the administration to boost security aid to Ukraine, including by sending lethal maritime equipment to Kiev. Some have also suggested the U.S. and other European partners increase their naval presence in the Black Sea to conduct patrols or routine training exercises. Others have suggested additional sanctions could be leveled to further squeeze Moscow.

“It’s really important that the United States takes a firm stance,” said Evelyn Farkas, who served as deputy assistant secretary of defense for Russia, Ukraine and Eurasia during the Obama administration.

“These types of strong men, they only stop when they are forced to, when the international community says this is unacceptable and you have to stop and make the price they have to pay too high,” said Farkas, now a senior fellow at the German Marshall Fund.

Trump will have a platform at the G-20 to call Russia out for its behavior.

“We want the administration, our allies to really press Russia on this. This is completely unacceptable and should be condemned,” Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' The Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination MORE (R-Wis.) said. “We need to really react to this with strength and resolve or Putin will keep pushing.”

The Ukraine issue is one that also plagued the Obama administration, which was first to grapple with Russia’s annexation of Crimea. Some criticized former President Obama for being reluctant to send lethal defensive aid to Ukraine, which Trump has done.

“The big challenge is to send a clear signal to the Russian federation that this kind of aggression has a red line. This is not something that the previous administration, the Obama administration, did effectively,” said Alina Polyakova, a foreign policy expert at the Brookings Institution. “This is an opportunity for this administration to send a clear message by taking specific actions, whether it be more broad, painful economic sanctions, where it be sending U.S. vessels into the Black Sea.”

Tensions have simmered in the region for four years, but the encounter at sea represents an escalation that experts say could worsen if left unchecked by the U.S. and other western powers.

Ukraine voted to impose martial law across a wide swath of its territory on Monday amid the heightening tensions and has asked NATO to send warships to the Sea of Azov for additional security.

In a recent interview with German media, Ukraine President Petro Poroshenko accused Putin of trying to annex his entire country. He has separately called on Trump to issue a firm message to Putin at the G-20 to “get out from Ukraine.”

Experts view the Kerch Strait incident as the latest effort by Putin to test the West.

The Trump administration has taken actions against Russia for the poisoning of ex-spy Sergei Skripal in Britain. It also charged a handful of Russian intelligence officers for their role in global hacking operations.

But those efforts have not always squared with the president’s rhetoric, creating a dissonance between him and his advisers that has left onlookers puzzled and at times skeptical of Trump’s views.

Trump’s friendliness toward Putin has been viewed against the backdrop of special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation into potential collusion between Trump’s campaign and Moscow, a probe that the president has increasingly castigated as a partisan witch hunt. On Thursday, Mueller secured the cooperation of Trump’s former personal lawyer Michael Cohen, a development that is likely to further rankle the president.

Trump endured a raft of criticism following the Helsinki summit with Putin in July at which he cast doubt on the U.S. intelligence community’s assessment of Russian interference — a comment he later walked back.

Some argue that Trump’s policies outweigh his public overtures toward Moscow.

“While his performance in Helsinki was abysmal, the policies coming out of his administration have been really good on Russia,” said Luke Coffey, a foreign policy expert at the Heritage Foundation. “If I was a Ukrainian soldier fighting on the frontlines in the Donbass and I had a choice between having a strongly worded tweet from President Trump about Russia or actually having a Javelin anti-tank missile, I would pick the anti-tank missile.”

The cancellation of Saturday’s planned meeting with Putin — which reportedly caught Moscow flatfooted — is likely to allow Trump’s advisers to avert a repeat of the Helsinki blow up. But the decision will not quell demands from Capitol Hill and elsewhere for more decisive action.

“I think they should respond in a very clear way that it’s unacceptable for Russia to try to block commerce by denying Ukraine access to its ports,” Sen. Ben Cardin Benjamin (Ben) Louis CardinPelosi hopeful COVID-19 relief talks resume 'soon' Congress must finish work on popular conservation bill before time runs out PPP application window closes after coronavirus talks deadlock MORE (D-Md.) said Wednesday, who suggested it would not be “enough” for Trump to cancel the meeting.

“I think he needs to take more direct action to show that we stand behind the integrity of free commerce on the seas,” Cardin said.