The UN special envoy for Syria has given warning that violence in the country is the worst he has seen since taking the job four years ago.

Staffan de Mistura's remarks on Wednesday came as the US and Russia again traded blame at the UN over the ongoing conflict.

"Civilians have been killed on a horrific scale - reports suggest more than 1,000 civilians in the first week of February alone," he told the UN Security Council.

"I have been now four years as the special envoy. This is as violent and worrying and dangerous a moment as any that I have seen in my time of tenure so far."

De Mistura mentioned all the countries now fighting in Syria, including the Turkish operation around Afrin and the Syrian government's continued bombardment of Eastern Ghouta and Idlib.

He talked of developments in recent days, including the US attack on Assad's forces near Deir Az Zor and Israeli air attacks in Syria including on Iranian targets. But both these operations were later defended by Nikki Haley, the US ambassador to the UN.

"The United States will always reserve the right to act in self-defence. The [Syrian President Bashar al-Assad] regime has become a front for Iran, Hezbollah, and their allies to advance the irresponsible and dangerous agenda for the Middle East," she told the Security Council.

She criticised Russia for failing to stop the Assad regime from bombing and gassing civilians, drawing a sharp response from Moscow's permanent representative to the UN, Vassily Nebenzia.

Nebenzia said the US and its allies should use their influence over the opposition groups to prevent violence.

Meanwhile, the UN's latest modest peace effort based on the outcome of a conference in the Russian city of Sochi seems now to be in doubt.

De Mistura wants to select members of a new committee to come up with a new constitution for Syria. But Syria's ambassador at the UN rejected that.

"Participants of the conference did not lend any authority for Mr De Mistura to set up this committee," said Bashar al-Jaafari, the Syrian diplomat.

Earlier on Wednesday, the first convoy of aid since November was delivered to Eastern Ghouta, a rebel-held enclave east of Damascus.

Diplomats point to a familiar and cynical pattern by the Syrian government. It is only when Syria is in the international spotlight, a small amount of aid is finally delivered.

Aid also reached Deir Az Zor, which was liberated in November last year from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

Abdirahman Meygag, deputy director of the World Food Programme in Syria, told Al Jazeera from Damascus on Thursday that his team visited the city two days earlier for the first time since 2014.

"The majority of the city is uninhabitable, 80 percent of the city has been destroyed," he said.

"We have seen that there is no electricity, the majority of people depend on generators. Water is not running. The sewage system is disfunctional."

He also said thet the majority of the people depended on foreign assistance.

"For a year and a half the UN has been doing airdrop operations in Deir Az Zor city. We kept the people alive there. However, the city needs more than that and we are there to assist. We need to step up our operations."