British officials said Wednesday they have asked the United Nations to expand its investigation into possible chemical weapons use in Syria to three additional towns where rebels claim nerve agents were used by the government.

In advance of a peace conference planned in the coming weeks, Britain has sought to increase pressure on Syrian President Bashar al-Assad and his chief patron, Russia, to accept a transitional government and end the civil war.

On Monday in Brussels, Britain joined France in leading a successful push to block the extension of a European arms embargo on Syria, raising the prospect that European governments might ship arms to the rebels if political talks fail.

In an apparently choreographed move, British officials told reporters here Wednesday about their latest concerns over possible chemical weapons use. Britain and France have previously said they have evidence the government used chemical weapons. The Syrian government has denied the accusations, countering that the rebels used the banned weapons.

In a request delivered last week, British Ambassador Mark Lyall-Grant urged U.N. Secretary General Ban Ki-moon to investigate rebel claims that Syrian forces used chemical weapons in March in the town of Adra, near Damascus; in Darayya in April; and in Saraquib in late April, according to a diplomat familiar with the British account who spoke on the condition of anonymity to discuss internal communications.

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A spokeswoman for Britain’s U.N. mission, Iona Thomas, declined to discuss the details of the claim. But she said, “The United Kingdom’s permanent representative to the United Nations has written to the U.N. secretary general to draw attention to three further allegations of chemical weapons use in Syria and have asked that they be included in the U.N.’s ongoing investigation.”

In March, the Syrian government invited the United Nations to investigate possible chemical weapons use in an incident that killed 26 people, including government troops. Britain and France countered with calls for investigations into the possible use of chemical weapons by the Syrian government.

Ban appointed a veteran Swedish chemical weapons expert, Ake Sellstrom, to investigate the allegations. But Syria has not agreed to permit an investigation into the European claims and has blocked the inspectors from entering the country.

As an alternative, Sellstrom is seeking to collect as much evidence as possible outside the country, interviewing government officials with access to intelligence on Syria’s chemical weapons program, refugees and other potential eyewitnesses who have fled.