US President Donald Trump says he may cancel a meeting with Russian President Vladimir Putin following a maritime clash between Russia and Ukraine, reports agencies.
Mr Trump told the Washington Post he was waiting for a "full report" after Russian ships fired on and seized three Ukrainian boats on Sunday.
Ukraine described it as an "act of aggression" but Russia said the ships had illegally entered its waters.
Martial law has been imposed in parts of Ukraine, in an unprecedented move.
Meanwhile, the US has urged European states to do more to support Ukraine.
State department spokeswoman Heather Nauert said Washington wanted to see tougher enforcement of sanctions against Russia.
Russia will deploy new S-400 surface-to-air missile systems on the Crimean peninsula soon, the Interfax news agency cited the Russian defence ministry as saying on Wednesday.
News of the deployment comes after Ukraine introduced martial law for 30 days in parts of the country following Russia's seizure of three Ukrainian navy vessels off the coast of Russian-annexed Crimea on Sunday.
The RIA news agency said the new S-400 systems would be operational by the end of the year.
Mr Trump and Mr Putin are due to meet on the sidelines of the G20 summit in Buenos Aires later this week.
However, Mr Trump told the Washington Post that the report coming from his national security team would be "very determinative".
"Maybe I won't have the meeting [with Mr Putin]. Maybe I won't even have the meeting. I don't like that aggression. I don't want that aggression at all," he said.
The two men are scheduled to discuss security, arms control, and issues in Ukraine and the Middle East when the summit convenes on Friday and Saturday, national security adviser John Bolton told reporters.
Russian coastguard ships opened fire on Sunday as the two Ukrainian gunboats and a tug sailed through the Kerch Strait off the coast of Crimea, which was annexed by Russia in 2014. Twenty-four Ukrainians were detained and at least three were wounded in the incident.
A Crimean court later ordered that 12 of the Ukrainians be detained for 60 days. The court is expected to issue rulings for the remaining servicemen on Wednesday.
The FSB security service has since released videos of some of the men making statements. One of them, Volodymyr Lisovyi, said he was aware of the "provocative nature" of the Ukrainian action.
Another, Andriy Drach, said he had been on a gunboat with an order to sail from Odessa to Mariupol.
"We were warned by the border service of the Russian Federation that we were violating Russian law. They had repeatedly asked us to leave the territorial waters of the Russian Federation," he said.
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