European Council President Donald Tusk says the EU is willing to help British Prime Minister Theresa May win the British parliament's approval of their Brexit deal.
His tweet came after the two met in Brussels on Tuesday, but he questioned how the bloc could help.
May is on a desperate mission to get reassurances from EU leaders that the sticking point of the so-called "backstop" is only temporary.
The customs arrangement would prevent a hard Irish border after the UK leaves the EU in case no future trade deals are made.
But May's critics suggest it would keep the UK permanently under EU customs rules.
Other EU leaders have said the Brexit deal cannot be renegotiated, fueling concerns Britain will leave the bloc without a deal.