Russia has told the United States it has no intention of invading Ukraine and expressed concern about the build-up of American and Nato forces in eastern Europe.
Four British Typhoon fighter jets have arrived at a Nato base in Lithuania to take part in increased Baltic border patrols, while 600 US troops were deployed to Poland and the Baltic countries of Estonia, Latvia and Lithuania, reportedly for military exercises.
It comes amid continuing violence in eastern Ukraine, including in the city of Luhansk where hundreds of pro-Russian separatists stormed the regional government headquarters, unopposed by police.
The Kiev government said it had information the activists also planned to seize the local television centre.
Meanwhile, the US and European Union imposed more sanctions on Russian firms and individuals close to President Vladimir Putin.
Japan has also imposed visa bans on 23 Russian officials.
PIC: A child looks at pro-Russian militants standing guard in Konstantinovka
Moscow denounced the new US sanctions, accusing Washington of using Cold War tactics from a “bygone era”.
It also criticised the new European measures, saying the EU was simply doing Washington’s bidding and should be ashamed of itself.
In an hour-long phone call with Pentagon chief Chuck Hagel, Russian defence minister Sergei Shoigu voiced concern about an “unprecedented” increase in US and Nato activity near Russia’s borders.
Allies of Mr Putin have been subjected to visa bans and asset freezes, including his friend Igor Sechin, head of oil giant Rosneft.
US officials said America’s penalties, which affect seven individuals and 17 companies, were aimed at “cronies” of the Russian leader.
President Barack Obama said: “The goal is not to go after Mr Putin personally.
Pro-Russians attack a pro-Ukrainian protester during a rally in Donetsk
“The goal is to change his calculus with respect to how the current actions he’s engaging in Ukraine could have an adverse impact on the Russian economy over the long haul.”
The EU has added 15 more Russians and Ukrainians to its blacklist, including General Valery Gerasimov, chief of the Russian general staff, and Lieutenant General Igor Sergun, head of GRU, the Russian military intelligence agency.
Russian deputy prime minister Dmitry Kozak and pro-Russian separatist leaders in Crimea and the eastern Ukrainian cities of Lugansk and Donetsk were also on the list.
Gennady Kernes, the mayor of Ukraine’s second city, Kharkiv, who was shot in the back while exercising, is in a stable condition after being flown to Israel for treatment.
Pro-Russian separatists in Slavyansk continue to hold seven monitors from the Organisation for Security and Co-operation in Europe.
Russia has been accused by the US of stirring up unrest in Ukraine, allegations Moscow rejects.
PIC: A child looks at pro-Russian militants standing guard in Konstantinovka
Pro-Russians attack a pro-Ukrainian protester during a rally in Donetsk