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- Video report by ITV News Correspondent Paul Davies
US Secretary of State John Kerry has accused Russia of an "outrageous two-hour attack" which killed 20 UN aid workers on Monday.
Speaking at a UN Security Council meeting on Wednesday, Kerry said the attack dealt a blow to any hopes of peace.
Hopes that were hampered further after 49 people were reported to have been killed in Aleppo on Wednesday.
The United Nations has confirmed they will resume humanitarian aid convoys within Syria after an attack prompted a temporary suspension.
The air strike near Aleppo on Monday evening targeted a convoy delivering aid to 78,000 civilians.
Twenty UN aid workers and one civilian were killed in the assault that caused a global outcry.
The Syrian government has agreed to continue peace talks without preconditions, and insisted the country "will not become another Libya or Iraq".
Heeding an appeal from US Secretary of State John Kerry earlier on Wednesday afternoon, Syrian UN Ambassador Bashar al-Ja'afari told the UN Security Council his country intends to reach a political solution.
He said: "My country is ready to resume intra-Syrian dialogue with no preconditions and according to decisions and foundations that launched this very dialogue, in order to reach a political solution that is decided by the Syrians".
Russian Foreign Secretary Sergey Lavrov has dismissed claims his country was involved in the killing of 21 UN aid workers in Syria on Monday evening.
Speaking at the UN Security Council meeting on Wednesday, Lavrov said a US 'Predator-type' drone was responsible - adding that one left Incirlik Air Base in Turkey and flew across the same area.
He said any assumption Russia was involved is being used to distract attention away from the US-led coalition bombing of Syrian soldiers.
Lavrov's comments sparked an immediate reaction from UK Foreign Secretary Boris Johnson, who implied Russian involvement.
Johnson told reporters: "Put it this way, when you look at what happened to the aid convoy, there are only two possible culprits - there are only two forces capable of carrying out that strike, flying in that area.
"They are the Syrians and the Russians and we have our doubts over the Syrian capability to fly at night. You're left with a pretty strong conclusion."
Boris Johnson has pleaded with the international community to create the conditions for a ceasefire in Syria.
After attending a special UN Security Council meeting on the war-torn country, the Foreign Secretary told reporters: "I think nobody should be under any illusions.
"It's up to people in that room [the UN Security Council] to create the conditions for a ceasefire. They've done it before, they can do it again.
"We know the overwhelming responsibility for the failure to have a ceasefire...lies with the Assad regime and indeed its sponsors. The Kerry-Lavrov process is in a pretty critical state but it is not yet terminal and it can be revived.
"We owe it to the people of Syria."
The US Secretary of State has called for immediate no-fly zones to be implemented in Syria and warned the future of the country hangs by a thread.
John Kerry said: "I believe that to restore credibility to the process [of creating a ceasefire], we must move forward to try to immediately ground all aircraft flying in those key areas [in Syria], in order to de-escalate the situation, and give a chance for humanitarian assistance to flow unimpeded.
"In Geneva, Russia [said] that Assad was prepared to live by the cessation of hostilities and would accept the idea of not flying over agreed upon areas.
"Because of what's happened over the last few days, we have to... move immediately... to implement a genuine ceasefire now.
"The future of Syria is hanging by a thread. I call on every country to cease providing support of any kind to any party that is trying to sabotage this plan", he added.
The US Secretary of State has said the "moment of truth" has come for a ceasefire in Syria.
Addressing the UN Security Council on Wednesday, John Kerry said: "The primary question is no longer what do we know, the primary question is collectively what are we going to do about it.
"This is a moment of truth. It's a moment of truth for Putin, and it's a moment of truth for the opposition - and those who support the opposition.
"It's a moment of truth for the international community.
"If we allow spoilers to choose the path for us.. make no mistake, the next time we convene here [at the UN Headquarters in New York], we're going to face a Middle East with even more refugees, with more dead, with more displaced, with more extremists and more suffering on an even greater scale."
Parties in the Syrian conflict are committing "flagrant violations" of international law.
US Secretary of State John Kerry told the UN Security Council: "You don't need to read these documents to understand it's against international law to bomb hospitals.
"You don't need these documents to understand you don't drop barrel bombs on children".
Mr Kerry then described the death of 20 aid workers on Monday as an "outrageous, sustained, two-hour attack" on a "fully-authorised" humanitarian mission.
He said the attack had dealt a "very heavy blow" to efforts to bring peace to Syria.
The US Secretary of State has slammed his Russian counterpart during a blistering attack at the UN Security Council meeting in New York.
John Kerry appeared to suggest Sergey Lavrov was a "spoiler" who "shredded" any hopes of a ceasefire.
He said: "I listened to my colleague from Russia - and I sort of felt [we're] in a parallel universe here.
"He [Sergey Lavrov] said that nobody should have any preconditions to come to the table. Well, we met in Vienna twice. We met in New York, and embraced the United Nations security council resolution.
"We met again in Munich. And in each place, the international Syria support group - and here in the UN Security Council - embraced a ceasefire applicable to all parties.
"That's not a precondition. That's an international agreement. Four times arrived at. Four times countries have said 'we will do this'. And four times it's been shredded by independent actors, by spoilers who don't want a ceasefire."
John Kerry criticised parties willing to engage in diplomatic relations with the Syrian government.
The US Secretary of State said: "How can people go sit at a table with a regime that bombs hospital and drops chlorine gas, again and again and again, and acts with impunity?
"Are you supposed to sit there and have happy talk in Geneva under those circumstances when you have signed up for a ceasefire?"
Mr Kerry asked what credibility that left those countries with.
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It said "armed terrorist groups" had repeatedly violated the brokered agreement which came into force last week.