"I think that, in the UK, we need a very concrete understanding of what the UK really wants. To stay one leg on the continent and one leg in the UK is not possible," Grybauskaitė told reporters as she arrived in Brussels for an EU summit on Wednesday.
"Of course, we'll be ready to compromise if we know what the UK really wants," she said, adding, "We have time. I think the UK has less time."
There have been growing fears in Brussels and London lately that the UK might have to leave the EU next March without a deal as the two sides fail to agree on several key issues.
Grybauskaitė said a no-deal Brexit would not be "a big loss" to Lithuania, because "what matters to us in this agreement are only the rights of our citizens living in the United Kingdom".
EU diplomats said on Wednesday that the bloc was ready to extend the "post-divorce" transition period for London.
Negotiations are stuck over the so-called "backstop" arrangement to avoid a hard border between British Northern Ireland and EU member Ireland after the UK leaves the bloc.
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