UNITED NATIONS (Reuters) - Britain pressed Syria and its ally Russia on Thursday to allow access to humanitarian aid in the war-torn country as a senior U.N. official told the Security Council that aid convoys had not been able to reach 540,000 people in besieged areas during July.

Britain's Ambassador to the United Nations Matthew Rycroft delivers remarks during the Security Council meeting on the situation in Syria at the United Nations Headquarters, in New York, U.S. April 7, 2017. REUTERS/Stephanie Keith

“We’re not asking for humanitarian access as a favor. We’re asking for it because it’s a legal and moral obligation,” Britain’s U.N. Ambassador Matthew Rycroft told reporters. Russia should use its influence over Syrian President Bashar al-Assad to ensure access to aid, he said.

Syrian U.N. Ambassador Bashar Ja’afari accused U.N. Secretary-General Antonio Guterres of slandering the Syrian government in his monthly report to the Security Council on aid access.

“Humanitarian aid convoys very often go to armed terrorist groups instead of reaching the Syrian people who need them. This only aggravates the crisis, it only encourages terrorists who use civilians as human shields,” Ja’afari told the council.

The deputy U.S. ambassador to the United Nations, Michele Sison, responded: “The Syrian government continues to terrorize the Syrian people and cause unimaginable suffering.”

Fourteen Geneva-based ambassadors, including the United States, Britain and France, have sent a letter to the Security Council to demand that it take action to ensure aid convoys reach millions of needy Syrians.

The 15-member council has long been divided on how to end the war, with Russia - backed by China - pitted against the United States, France, Britain. Those five countries are veto-wielding Security Council members.

Russia has vetoed eight resolutions on Syria to shield Assad’s government from action, including a referral of the situation in Syria to the International Criminal Court for a possible investigation into war crimes, genocide and crimes against humanity. China has backed Russia to veto six resolutions.

The humanitarian situation in many parts of Syria remains extremely difficult, said deputy U.N. aid chief Ursula Mueller, despite a deal in May by Russia, Turkey and Iran to establish several de-escalation zones to allow access to aid.

“There have been no convoys to besieged areas in July,” Mueller said.

Obstacles included lack of approvals, lack of facilitation letters by the Syrian government and administrative delays, as well as insecurity and fighting, she told the council.