British Foreign Minister William Hague says Europe should continue its "strong and united" response to Russia's escalation of the Ukrainian crisis.
Hague spoke Friday before a meeting of European Union foreign ministers in Athens, where sanctions against Russia are expected to be on the agenda.
He said there has only been a "token withdrawal" of the troops massed near the Ukrainian border, whose presence has raised fears of a Russian invasion.
"We haven't seen real de-escalation by Russia and therefore Europe must not relax in preparing a third tier of sanctions and making sure we continue to have a strong and united response,'' he said, referring to tough trade and economic measures that the EU has threatened to take against Russia if it moves beyond Crimea into southern and eastern Ukraine.
Western nations have already passed sanctions against Russia because of its annexation of the Crimean peninsula, a move rejected by the U.S. and its allies.
Russia moved into the peninsula last month following the ouster of Ukraine's Moscow-backed President Viktor Yanukovych. The ouster followed weeks of opposition protests.
Russia blamed for protester deaths
On Thursday, Ukraine's new Western-friendly leaders blamed Russian agents and Yanukovych for two days of bloodshed during the protests that saw more than 100 people killed.
The mystery of the Kyiv protesters' deaths in February has fueled talk of conspiracies. Russian President Vladimir Putin has suggested the demonstrators themselves were the gunmen, acting to discredit the government of then-president Yanukovych.
Acting Interior Minister Arsen Avakov linked special police from its Ministry of Internal Affairs, acting under orders of Yanukovych, to the killing of at least 17 protesters. He said 12 "Berkut" police have been identified and that authorities have begun arresting them.
For his part, Ukraine's security service chief said evidence shows Russian agents were involved in "the planning and implementation" of the deadly February 18-20 police operations.
Some information for this report provided by Reuters.
People stand outside a closed McDonalds restaurant. The fast food restaurant chain announced this week that it is shuttering its three outlets in the Crimean peninsula over unspecified operations issues, Simferopol, Crimea, April 4, 2004.
People gather outside a currency exchange office in the Crimean city of Simferopol, April 4, 2014.
People stand in line as they wait to enter a branch of the Sberbank of Russia bank in the Crimean city of Simferopol, April 4, 2014.
Ukrainian Prime Minister Arseniy Yatseniuk said during an interview with Reuters that the Kyiv government will stick to unpopular austerity measures "as the price of independence" as Russia steps up pressure on Ukraine to destabilize, Kyiv, April 3, 2014.
Ukraine's acting President Oleksandr Turchynov visits a military exhibition near the settlement of Desna in Chernigov region, Ukraine, April 2, 2014.
Ukrainian soldiers watch as an army medic helicopter flies above during a military exhibition near the settlement of Desna in Chernigov region, April 2, 2014.
People pass by barricades near the Dnipro Hotel in Kyiv, April 1, 2014.
Self-defense activists pass by the Dnipro Hotel in Kyiv, April 1, 2014.
Members of the Ukrainian far-right radical group Right Sector leave their headquarters in Dnipro Hotel as police special forces stand guard, Kyiv, April 1, 2014.
Commuters walk along railway lines next to Ukrainian tanks ready to depart from Crimea near Simferopol, March 31, 2014.
Russian Prime Minister Dmitry Medvedev visits Crimea to consider priorities for its economic development, Simferopol, March 31, 2014.
Ukrainians, in accordance with Orthodox Church tradition of marking the 40th day since death, remember those who lost their lives during pro-Europe protests in Kyiv, March 30, 2014.