The United States on Monday warned Russia that "outlaw actions" like the seizure of Ukrainian ships in the Sea of Azov are preventing normal relations from developing between Washington and Moscow.
US Ambassador Nikki Haley told an emergency meeting of the UN Security Council that "the United States would welcome a normal relationship with Russia. But outlaw actions like this one continue to make that impossible."
Tensions have flared after the Russian navy on Sunday boarded and seized three Ukrainian ships off the coast of Crimea, accusing Ukraine of illegally entering Russian waters in the Sea of Azov.
The confrontation at sea raised fears of a wider military flareup.
Haley put the blame firmly on Russia for the clash, calling the seizure of the ships an "outrageous violation of sovereign Ukrainian territory" and slamming "yet another reckless Russian escalation."
But she did not specifically threaten further sanctions, instead calling for a de-escalation of tensions over Crimea, which Russia annexed in 2014 in a move never recognized by the international community.
"The United States will maintain its Crimea-related sanctions against Russia. Indeed, further Russian escalation of this kind will only make matters worse," said Haley.
"It will further undermine Russia's standing in the world. It will further sour Russia's relations with the US and many other countries. It will further increase tensions with Ukraine. "
Haley said her remarks followed conversations with President Donald Trump and Secretary of State Mike Pompeo and reflect "concerns at the highest level of the American government."
The US warning comes ahead of a planned meeting between Trump and his Russian counterpart Vladimir Putin, on the sidelines of the G20 summit in Argentina later this week.
Russia denies 'mythical' aggression
Russia's envoy accused Ukraine of provoking the confrontation to justify the imposition of martial law and a delay of elections as polls show Ukrainian President Petro Poroshenko faces possible defeat.
Ukraine's parliament was to vote Monday on a request from Poroshenko to impose martial law for 60 days.
Denouncing a "mythical Russian aggression," Russian Deputy Ambassador Dmitry Polyanskiy told the council: "Our country has never caused the first blow, but it knows how to protect itself."
Ukraine called for a toughening of sanctions against Russia, with Ambassador Volodymyr Yelchenko warning that "if unopposed, the Kremlin regime will take another and another aggressive act" leading to wider conflict.
The Ukrainian ambassador cited audio recordings of communications among Russian vessel commanders that showed they were given orders to attack the ships after they had changed course and were returning to their Ukrainian port.
France and Germany, which lead European diplomatic efforts to end the conflict in Ukraine, urged Russia to hand over the seized ships and release the seamen.
At the outset of the meeting, Russia suffered a setback in the council after it sought to discuss the clash in the Azov under an agenda item that described the incident as a violation of Russia's borders.
This was rejected in a procedural vote, with only China, Bolivia and Kazakhstan siding with Russia. The council then discussed the clash under a less contentious agenda item.