Russia's deputy foreign minister raised concerns over what he said was increasing Black Sea naval activity by powers that did not have a coastline in the region, an apparent reference to the United States.
The United States will send two warships to the Black Sea next week, Turkey has said as Russia, which has beefed up its military forces near Ukraine, accused non-coastal NATO powers of increasing naval activity in the region.
Washington says Russia has amassed more troops on Ukraine's eastern border than at any time since 2014, when it annexed Crimea from Ukraine and backed pro-Russian separatists in the eastern Donbass region of Ukraine.
Russian President Vladimir Putin accused Ukraine of "dangerous provocative actions" in its eastern Donbass region in a phone call with Turkish President Recep Tayyip Erdogan, the Kremlin said on Friday.
Violence has flared between Ukrainian troops and the separatists, spurring fears of a major escalation.
Turkey, a NATO ally, said the United States would deploy two warships to the Black Sea from April 14-15.
"A notice was sent to us 15 days ago via diplomatic channels that two US warships would pass to the Black Sea, in line with the Montreux Convention. The ships will remain in the Black Sea until May 4," Turkey's Foreign Ministry said.
The 1936 Montreux accord gives Turkey control over the Bosphorus and Dardanelles straits, which connect the Mediterranean to the Black Sea. It also limits access of naval warships and governs foreign cargo ships.
Russian troop build-up
Ukraine and the West have accused Russia of sending in troops and weapons to help separatists, accusations that Moscow has denied.
Western and Ukrainian officials have raised concerns in recent weeks about increasingly frequent cease-fire violations in the country’s industrial heartland, known as Donbass. They also expressed worries about the Russian troop buildup along the border with Ukraine.
During a call with Putin on Thursday, German Chancellor Angela Merkel “called for the removal of these troop reinforcements in order to achieve a de-escalation of the situation.”
White House press secretary Jen Psaki said on Thursday the US is also increasingly worried about the Russian troop buildup, noting that Russia now has more troops on the border with Ukraine than at any time since 2014.
In response, Peskov said Russia is free to deploy its troops wherever it wants on its territory. He accused the Ukrainian military of an “escalation of provocative actions” along the line of control in the east that threatens Russia's security.
“The Kremlin has fears that a civil war could resume in Ukraine. And if a civil war, a full-scale military action, resumes near our borders that would threaten the Russian Federation's security,” Peskov said. “The ongoing escalation of tensions is quite unprecedented.”
In Kyiv, Colonel-General Ruslan Khomchak, the commander-in-chief of Ukraine's armed forces, rejected Moscow's claims of the alleged Ukrainian preparations for an offensive in the east as part of a “disinformation campaign” and a “hybrid war.”
Dmitry Kozak, a Putin aide who serves as Russia’s top negotiator with Kyiv, warned Ukraine on Thursday against using force to retake control of the east, where many residents have Russian citizenship. Such a move would mark “the beginning of an end for Ukraine,” he said, adding that Russia would likely act to protect its citizens.
Asked about Kozak’s comment, Peskov alleged that virulent nationalist rhetoric in Ukraine was inflaming hatred against the mostly Russian-speaking population of the east. He claimed that if civilians in eastern Ukraine faced the threat of a massacre, “all countries, including Russia, will take steps to prevent such tragedies.”
Russian Deputy Foreign Minister Alexander Grushko raised concerns on Friday over what he said was increasing Black Sea naval activity by powers that did not have a coast line in the region, an apparent reference to the United States.
"The number of visits by NATO countries and the length of the stay of (their) warships have increased," he was quoted as saying by the Interfax news agency.
Such visits by the US and other NATO ships have vexed Moscow, which long has bristled at Ukraine's efforts to build up defence ties with the West and its aspirations to eventually join NATO.
According to a Reuters witness who keeps track of ships passing through Turkey's Bosphorus strait, the United States and NATO increased their presence in the Black Sea early this year, when US President Joe Biden's administration took power.
The Reuters witness said the level had reached that seen in 2014-2015 at the time of the Crimea annexation.
Zelenskiy is due to meet Erdogan in Turkey on Saturday on a previously scheduled visit.
Ukraine rules out offensive
Ukraine's army said it would not launch an offensive against pro-Russian separatists controlling two regions in the east.
"The liberation of the temporarily occupied territories by force will inevitably lead to the death of a large number of civilians and casualties among the military, which is unacceptable for Ukraine," Ruslan Khomchak, chief of the general staff of the Ukrainian Armed Forces, said in a statement on Friday.
He accused Moscow of using "intimidation and blackmail by military force" to exacerbate the situation.
"Ukraine is supported by the entire civilised world. We are not alone in the face of the enemy," Khomchak added.
Zelensky on the frontline
Amid the intensifying fighting, Zelenskiy on Thursday visited the eastern frontline and spent the night there.
"Our country's fate is determined here," Zelensky said on Facebook on Friday, while images released by his office showed him in the trenches clad in a helmet and bulletproof vest, shaking hands with soldiers.
"These are the points of escalation", the president said, adding that he had conversations with soldiers "to the sound of gunshots".
Zelenskiy, who has urged NATO to speed up his country's membership into the alliance to support Ukraine, said he had visited positions where Ukrainian troops were killed and wounded in recent weeks.
He said 26 Ukrainian soldiers had been killed since the start of the year, compared to 50 in all of 2020, when fighting in the conflict subsided as a new ceasefire agreement took hold last July.
But clashes, mainly involving artillery and mortar fire, have picked up again since the start of the year, with both sides blaming each other.
Separatists said that more than 20 of their fighters had been killed in 2021, while the conflict in Ukraine's mainly Russian-speaking east has claimed more than 13,000 lives since it erupted.