The British Museum is guarding a precious artefact that was looted from Syria, in the hope of returning it when the country is stable.
Director Neil MacGregor told The Times the museum was trying to protect antiquities taken from conflict zones.
And he called on the government to sign up to an international convention to protect cultural artefacts.
"We are playing a significant part in holding objects that have been illegally exported," he said.
"We did that in Afghanistan and are now returning them. We are holding an object we know was illegally removed from Syria and one day it will go back."
The destruction of museums, artefacts and archaeological sites amidst the Syrian conflict has been called "the worst cultural disaster since the Second World War".
Islamic State (IS) militants have been accused of looting and selling Syria's cultural heritage to raise funds, leading to a UN ban in the trade of artefacts from the country.
There are fears IS may destroy the 2,000-year-old Roman-era ruins in Palmyra after militants over-ran the town last week.
Syrian officials said they had moved hundreds of Palmyra's statues to safety prior to the IS takeover but could not transfer large monuments.
But across Syria, volunteers have risked their lives to preserve and protect irreplaceable monuments and mosaics, many of which date back to the 1st and 2nd Century; while civilians have turned over thousands of ancient artefacts for safekeeping.
The British Museum said it could not reveal which Syrian artefact it was holding.
"The question of whether we should help with the conservation and preservation of another country's heritage is serious," MacGregor said.
"There is a significant international agreement, the Hague Convention about the protection of cultural property in war zones. Only one country in the security council hasn't ratified it, the UK."
Several leading figures have called on the government to ratify the convention, while Labour's shadow culture secretary, Chris Bryant, says the party is committed to prompt ratification.
"I am at a loss to understand why the government has not done so. We shall be pushing them on this throughout this parliament," he said.
A spokesman for the Department for Culture, Media and Sport said the government would seek to legislate on the convention "as soon as parliamentary time allows".