Several EU leaders offered clarifications on Thursday to help British Prime Minister Theresa May sell the Brexit deal to lawmakers in London, while insisting that the text itself cannot be reopened, ahead of summit talks aimed at breaking the deadlock.

May's trip to Brussels comes in a tumultuous week that has seen her delay a pivotal Brexit vote in parliament to avoid defeat, before surviving a leadership challenge that has left her further weakened.

The premier said she would ask her EU counterparts for "legal and political assurances" to respond to the concerns of parliamentarians intent on voting against the Brexit deal because they fear it will surrender too much sovereignty to Brussels.

However she played down expectations of Thursday's talks.

"I don't expect an immediate breakthrough but what I do hope is that we can start to work as quickly as possible on the assurances that are necessary," May said.

Time is running short to ensure a smooth British departure from the European Union on March 29, 2019.

Ahead of the summit, German Chancellor Angela Merkel ruled out reopening the withdrawal agreement negotiated with London, noting that the EU will abide by its "principles."

"Of course it's possible to talk about whether there should be additional assurances, but here also the 27 member states will act as one and (...) make their interests clear," she added.

Dutch Prime Minister Mark Rutte said the 27 other EU countries can help to "demystify" a controversial clause known as the Irish backstop.

The backstop is a key point of contention for eurosceptic lawmakers but is seen by Brussels and many British observers as an essential fallback to ensure no hard border emerges between Ireland, one of the 27, and Northern Ireland, which is part of the United Kingdom, amid fears of a return to sectarian violence in the region.

Meanwhile, Austrian Chancellor Sebastian Kurz said there was still "room for manoeuvre" to resolve the Brexit standoff.

At the summit, May is scheduled to discuss the domestic challenges she faces with her EU counterparts, before they withdraw for dinner discussions on how far they can go to help.

The 27 EU leaders are expected later to issue a joint statement. An early draft contained no new concessions to London, sources in Brussels told dpa.

The leaders could give assurances that the backstop - which could leave Northern Ireland with a slightly different arrangement from the rest of the UK - would ideally never be used and, if so, then "only for as short a period as possible," said Irish Prime Minister Leo Varadkar.

Eurosceptic lawmakers in Britain fear that the backstop could lock the country into a close relationship with the EU that London cannot end unilaterally.

In the draft statement, leaders pledge their "firm determination" to work on negotiating a future partnership that would obviate the backstop by the end of 2020.

However, there is no indication that they could offer to limit the duration of the backstop, as some in London are demanding.

"If the backstop has an expiry date, if there is a unilateral exit clause, then it's not a backstop," Varadkar said, following pre-summit talks with May.

While some of May's suggestions "made sense," others were "difficult," the Irish premier noted.

Thursday's summit statement could be followed up with something more at a later date, an EU source said on condition of anonymity, without going into detail.

At stake is the withdrawal agreement, which details the terms of Britain's departure from the EU in March, and a political declaration outlining London and Brussels' ambitions for the future relationship.

Failure to finalize the withdrawal deal in time for Britain's departure could lead to an unregulated Brexit, which would likely cause mayhem for citizens and businesses on both sides of the border.

EU leaders are also due on Thursday to discuss their preparations for a no-deal scenario.