* EU looks at possible police mission to Ukraine
* Discusses more sanctions against Russia
* Four Ukrainians added to sanctions list (Updates with quotes from ministers)
By Justyna Pawlak and Adrian Croft
LUXEMBOURG, April 14 (Reuters) - The European Union threatened Russia with more sanctions on Monday over its actions in eastern Ukraine, which Britain said was being destabilised by Moscow, although some EU states said diplomacy should be given more time.
Ukraine dominated talks among EU foreign ministers after Kiev threatened military action against pro-Russian separatists occupying government buildings in the east.
British Foreign Secretary William Hague, arriving for the meeting in Luxembourg, said there could were was no doubt that Moscow was behind the destabilisation of eastern Ukraine.
"I don't think denials of Russian involvement have a shred of credibility," Hague told reporters, adding that the EU now needed to discuss adding more people to a list of 33 Russian and Ukrainian officials targeted by EU asset freezes and travel bans over the Ukraine crisis.
Polish Foreign Minister Radoslaw Sikorski said the EU must now agree how the list could be expanded.
"The EU has to make it clear to Russia what are the consequences of any possible future actions in eastern Ukraine," he said. "I expect a very specific signal when we can expect sanctions if Russia takes further steps."
But other governments were more cautious on sanctions, underscoring concerns in parts of Europe about antagonising a power with an energy stranglehold over the bloc, and put their faith in talks planned for Geneva on Thursday.
Germany said the planned meeting between the EU, the United States, Russia and Ukraine could help calm tensions even though the option of sanctions remained on the table.
In addition to widening asset freezes and visa bans, the EU is discussing possible more far-reaching measures, such as restrictions on trade and finance with Russia, which Hague said should be prepared quickly.
Sweden's Carl Bildt said Thursday's talks in Geneva could serve as a deadline for any further EU action.
"At the moment we have the Geneva meeting coming up, so that is an opportunity for (Russia) to start to de-escalate. If they continue to escalate I think we should escalate," he said.
Jean Asselborn, Luxembourg's foreign minister, said sanctions wouldn't "resolve the problem".
"Our main task today is to do everything so that this conference on Thursday in Geneva can take place and take place in a calm atmosphere," he said.
The ministers agreed some steps to help Ukraine overcome its deepening economic crisis, approving a package of nearly 500 million euros ($685 million) worth of trade benefits, which include the removal of duties on a wide range of agricultural goods, textiles and other imports.
Four Ukrainians were also added to a list of people targeted with EU sanctions over misappropriation of state funds.
Under discussion on Monday is also a possible EU mission to train police and other law-enforcement officials in Ukraine to help stabilise the country, as proposed by Britain, Sweden and Poland, and supported by Germany.
Such a mission - likely to irritate Russia - would seek to rebuild Ukraine's police and legal system in the short term to help combat violence in the country of 46 million people and lay the groundwork for implementing a proposed free-trade deal with the European Union.
The bloc will also discuss its approach to issuing visas and trade with the Crimean peninsula, which was annexed by Moscow in March after popular protests toppled pro-Russian president Viktor Yanukovich in Kiev.
"It is important to show that there won't be a recognition of the annexation through the back door," said one diplomat. (additional reporting by Tom Koerkemeier, Luke Baker and Barbara Lewis in Brussels; editing by Giles Elgood)