UNITED NATIONS (AP) — Britain said it has sent a letter to Secretary-General Ban Ki-moon with new information on three further incidents of alleged chemical weapons use by the Syrian government.

Britain's U.N. Ambassador Mark Lyall Grant said Wendesday his government has continued to provide new information to the secretary-general and the head of the U.N. team Ban appointed to investigate alleged chemical weapons attacks in Syria.

The Syrian government asked Ban to investigate an alleged chemical weapons attack by rebels on March 19 on Khan al-Assal village in Aleppo, but insists that a probe be limited to that incident. Syrian soldiers were reportedly killed and injured in the incident, which the rebels blame on Syrian forces.

Ban is insisting on a broader investigation, including a December incident in Homs raised by Britain and France. He appointed Swedish chemical weapons expert Ake Sellstrom to lead a U.N. investigation. Syria has refused to allow his team into the country.

"We continue to inform the secretary-general and Mr. Sellstrom of any information as and when we get it," Lyall Grant told several reporters. "I sent a further notification to the secretary-general last week."

The U.K. Foreign Office said the letter was meant to draw Ban's attention to three further allegations of chemical weapons use and ask that those be included in the U.N.'s investigation. Those allegations relate to incidents which reportedly took place in March and April of this year and which have been reported in the media, the Foreign Office added.

"The U.K. is extremely concerned about the ongoing allegations of chemical weapons use in Syria," it said in a statement.

A senior U.N. diplomat said last week that Ban has received new information about alleged chemical weapons incidents since the beginning of April. The diplomat, speaking on condition of anonymity because of the sensitivity of the issue, refused to give any details.

The confirmed use of chemical weapons could escalate the international response to the more than two-year-old conflict, which has killed more than 70,000 people, according to the United Nations.

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"Our view is that all the attacks are by the government," Lyall Grant said. "We have no information that the opposition has access, let alone used chemical weapons."

Last week, U.N. Mideast envoy Robert Serry told the Security Council that there were mounting reports of chemical weapons use as violence escalates in Syria. He gave no details but said the secretary-general remains "gravely concerned" about the allegations of chemical weapons use.

In response to the reports, Serry again urged the Syrian government to allow chemical weapons experts into the country immediately to investigate the allegations. While the U.N. team can question people outside Syria and analyze material they obtain, Ban has stressed repeatedly that on-site investigations are essential if the U.N. is to determine whether chemical weapons have been used.

U.S. Ambassador to the U.N. Susan Rice said American experts have briefed the U.N. team twice "and we will continue to provide relevant information as time goes on."

White House spokesman Jay Carney said the United States is working with its allies and the opposition to gather evidence about the potential use of chemical weapons.

"I think there's a concerted effort from a number of areas to try to gather information about this possible chemical weapons use, to document it, to prove it, and then to move forward once that process is ... completed," Carney said.

U.N. diplomats said France has provided additional information to the U.N. about the incidents in Khan al-Assal and Homs as well as a third incident in March near Damascus that Britain and France wanted investigated.

Rice also urged Syria to give the U.N. team unfettered access so it can investigate all allegations of alleged chemical weapons use.

"We're not surprised but indeed disappointed that the Syrians have reneged on their initial willingness to admit the investigation team, and thus the investigation remains stalled," she said.

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Vinograd reported from London. Associated Press writer Karin Laub contributed from Beirut.