Access for foreign aid for millions of Syrians desperately in need has not improved, violating a Security Council resolution passed two months ago, the UN chief said in a report Wednesday.
In February, the Security Council passed the first humanitarian resolution of the three-year civil war, demanding that the government and armed opposition allow aid to reach civilians.
But almost 3.5 million people remain without access to essential goods and services, including life-saving medicines, in a "clear violation" of international law, wrote Ban Ki-moon.
The report sent to the Security Council blamed both sides in the war but singled out the government for particular censure.
"The security situation is deteriorating and humanitarian access to those most in need is not improving. It remains an extremely challenging environment in which to work," said the document.
Thousands of people are not getting the medical care that they need -- which Ban described as "arbitrary and unjustified" and "a clear violation of international humanitarian law".
"I must again urge the parties, and in particular the government... to honor their obligations under international humanitarian law and act now," he said in the report.
Rebels and government forces have refused to lift sieges of populated areas and Ban said it was "shameful that nearly quarter of a million people are being deliberately forced to live under such conditions".
He called on the government and opposition to facilitate aid access and border and combat line crossings.
"People are dying needlessly every day," his report said.
The Syrian government has allowed aid convoys to cross only one of eight border crossings identified by the United Nations as a priority, Western ambassadors reported last month.
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- 'War escalates' -
Earlier on Wednesday, five UN agencies made an impassioned call on the warring parties to allow aid deliveries countrywide, end siege warfare and halt indiscriminate attacks on civilians.
"The war escalates in many areas. The humanitarian situation deteriorates day after day," the leaders of the UN humanitarian aid office and its children's, refugee, food and health divisions said in a joint statement.
In Aleppo alone, at least one million people are in need of urgent humanitarian assistance, but the road linking the northern city to the capital Damascus has often been cut.
In Aleppo, there are now only 40 doctors for a population of 2.5 million, where once there were more than 2,000.
And across Syria, more than 9.3 million people are now affected by the conflict which erupted in March 2011, the agencies said.
Last week, Security Council members held three hours of consultations after UN humanitarian chief Valerie Amos outlined the plight of Syrian civilians caught in fighting in Homs.
Britain and France proposed a draft formal declaration by the council to call for Syrian government forces to lift the siege, but the council failed to adopt it.
French ambassador Gerard Araud said that Russia, the key ally of Damascus, blocked the draft statement, despite Moscow's support for the February resolution on humanitarian access.
In a statement, American ambassador Samantha Power appealed "to all those UN member states with influence on Damascus to pressure the regime to return to the negotiating table."
"It is imperative that those who want to leave Homs are able to do so quickly and safely," she added.
But Syrian ambassador Bashar Jaafari told reporters at the time that only 170 civilians were trapped in Homs, alongside thousands of "terrorists" -- the regime's term for rebels.
He said the civilians refused to leave.