More from Chris Miller in Kyiv:

Russia's Foreign Ministry said it is currently investigating the incident and warned Kyiv of a harsh response. “If we are talking about taking Russians hostage, this would be qualified as a flagrant violation of international law and the consequences will come quickly," it said in a statement carried by state media.

The detention comes as negotiations over the release of the Ukrainian sailors were gaining momentum. It is unclear whether this may jeopardize a deal that Ukrainian Human Rights Ombudswoman Liudmyla Denisova said on July 23 was in the works.

"Agreements have already been reached on returning the sailors. There is a ruling by the International Tribunal for the Law of the Sea. The last documents are currently being finalized, and I think we will be greeting the sailors on home soil soon," Denisova told the Hromadske TV channel.

Reuters wrap on the ship:

Here's more on the seizure of a Russian tanker from RFE/RL's Ukrainian Service:

Ukrainian Authorities Seize Russian Tanker

Ukraine’s Security Service (SBU) says it has seized a Russian tanker that Moscow allegedly used to block three Ukrainian naval vessels before detaining them and 24 Ukrainian sailors in November near Crimea.

The SBU said in a statement posted on its Facebook page on July 25 that an investigation revealed that Russia's Federal Security Service and Border Guard Service used a tanker named Neyma to "illegally block the movement of the Ukrainian naval vessels Nikopol and Berdyansk, and the military tug Yani Kapu, in the Kerch Strait" before "illegally" detaining Ukrainian sailors and the three vessels.

According to the statement, the tanker, which has since changed its name to Nika Spirit, was seized by Ukrainian authorities after it arrived under the Russian flag at the Ukrainian port city of Izmayil on the Danube River on July 24.

When contacted by RFE/RL, Olena Hitlyanska, a spokeswoman for the SBU, said she could not yet share information about the sailors on board the ship.

On November 25, Russian forces fired on, boarded, and seized the three Ukrainian naval vessels near the Kerch Strait. Russia has been holding 24 Ukrainian sailors, who face up to six years in prison if convicted, since then.

Moscow claims the Ukrainian vessels illegally entered Russian territorial waters near Crimea, the Ukrainian peninsula that Russia now occupies after seizing it in 2014.

The Kerch Strait is the sole passage from the Black Sea to the Sea of Azov. It runs between Russia and Crimea, the Ukrainian peninsula that Russia took over by force in March 2014 after sending in troops and staging a referendum deemed illegitimate by Kyiv, the United States, and a total of at least 100 countries.

Russia moved swiftly to seize control over Crimea after Moscow-friendly Ukrainian President Viktor Yanukovych was pushed from power by the pro-European Maidan protest movement in February 2014.

Russia has also fomented unrest and backed separatists in eastern Ukraine, where more than 13,000 people have been killed in the ensuing conflict since April 2014.

With reporting by Christopher Miller in Kyiv

SBU has published video purportedly showing the ship's detention:

More from Chris Miller:

Reached by RFE/RL, Olena Hitlyanska, a spokeswoman for the SBU, said she could not yet share information about the sailors on board the ship.

From our Kyiv correspondent, Chris Miller, on the detained Russian ship/:

Russia's Foreign Ministry responds. It says it's investigating the incident. “If we are talking about taking Russians hostage, this would be qualified as a flagrant violation of international law and the consequences will come quickly."