UPDATE 1-EU says Belarus gas cut is attack on entire bloc

* EU energy chief blames Belarus for gas cut to Lithuania

* Oettinger says Belarus move is attack on entire EU

* Sees Ukrainian progress towards joining Energy Community

* EU reaches internal deal on gas security rules

(Adds quotes, detail, background)

By Pete Harrison

BRUSSELS, June 23 (Reuters) - Europe's energy chief warned Belarus on Wednesday against tampering with gas flows to Lithuania, describing a cut in Russian transit supplies to the EU as an attack on the entire bloc.

Belarus threatened earlier in the day to halt all Russian oil and gas flows to Europe if Moscow does not repay a debt for gas transit, raising the stakes in a dispute that threatens EU members Poland, Germany and Lithuania.

Lithuanian gas firm Lietuvos dujos (LDJ1L.VL) said it had already been hit by a 40 percent reduction by 1300 GMT, prompting European Energy Commissioner Guenther Oettinger to issue a warning.

"This is not only a problem for this one member state, it is an attack against the whole EU," Oettinger told reporters. "The government of Belarus wants to integrate Europe in their problems, and that is not OK."

In a sign of continued rapprochement with Moscow, he added: "We trust our Russian partners, that they want to bring their gas to Europe."

Oettinger spoke after a meeting with Ukrainian energy minister Yuriy Boyko, who pledged to help Europe overcome gas shortages.

"We shall do the best in order to help EU countries to avoid any problems in the energy sector," said Boyko.

Boyko was visiting Brussels to discuss how Ukraine could align its energy system more closely with that of Europe, patching up a relationship that was damaged by Kiev's own row with Moscow in 2009, which halted gas flow to the EU during 2 weeks of freezing weather.

Oettinger said Ukraine's overhaul of its gas laws was a positive step in its efforts to join Europe's Energy Community.

The Energy Community, an organisation seeking to liberalise the sector, provisionally accepted Ukraine as a member last December, a move that could help the country modernise its gas transit system.

The group, created in 2005 to ensure close cooperation within the EU and neighbouring countries, said Ukraine would be able to join when and if it made its gas laws compliant with those of the European Union.

"I think the government is moving in a strong way, step by step," said Oettinger.

The European Union also struck an informal deal among its members on Wednesday aimed at bolstering the security of energy supplies.

The deal, which still needs formal approval from the bloc's 27 member governments and the European Parliament, demands that countries ensure they are capable of coping if their main gas source fails during a time of heavy demand.

The rules also lay down minimum capacities for gas storage capacity, demand reversible gas flows and a commitment to break dependence on any single overseas supplier.

(Reporting by Pete Harrison; Editing by Keiron Henderson)