BRUSSELS, Nov 13 (Reuters) - The European Commission has formally authorised the temporary reimposition of border controls by Sweden and Germany's extension of frontier checks to help get a grip on a large influx of asylum seekers, the EU executive said on Friday.

With refugees flooding into European Union territory in the worst migrant crisis since World War Two, the viability of the EU's Schengen system of open borders is under strain as never before as EU members reintroduce controls at their frontiers.

Sweden, long a haven for people fleeing war and persecution, was the latest EU state to re-establish checks at its borders to stem a tide of migrants coming from Denmark.

Germany reimposed border controls on Sept. 13 and decided to extend them beyond an initial limit of two months foreseen by Schengen rules, using a clause that permits stretching checks to a maximum of six months.

"The temporary reintroduction of border controls between member states is an exceptional possibility explicitly foreseen in and regulated by the Schengen Borders Code, in case of a serious threat to public policy or internal security," the European Commission said in a statement.

The Swedish decision "prima facie appears to be a situation covered by the rules", it said.

The decision replicated similar provisional authorisations granted by the EU executive to Austria, Hungary and Slovenia when they imposed temporary border controls in recent weeks.

"We have no objections to measures that should remain proportionate," a Commission spokeswoman told a regular news briefing.

"(But) the situation serves to further underscore the pressing need to quickly implement the measures proposed in order to manage the refugee crisis," the statement said, including a plan to relocate refugees from the first countries of arrival to other EU states, thus reducing uncontrolled flows.

EU leaders held an extraordinary summit in Malta on Thursday to discuss the refugee crisis and stronger coordination with Turkey, the main transit country for migrants heading to Europe.

"Saving Schengen is a race against time. And we are determined to win that race," the head of EU leaders, Donald Tusk said after the meeting. (Reporting by Francesco Guarascio; Editing by Mark Heinrich)

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