The UK will tell other UN Security Council countries they must do more to tackle the humanitarian crisis in Syria.
International Development Secretary Justine Greening, who will chair the security council, will say that UN agencies helping people affected by the conflict are only 45% funded.
She will warn that time is "running out" for the people of Syria who have been "let down" by the international community.
The West and Russia are divided over the best approach for Syria and David Cameron will use a meeting with Vladimir Putin at the G20 summit in Turkey to voice the West's concern that Moscow's airstrikes are targeting moderate opponents of Syrian President Bashar Assad.
Ms Greening will tell the Security Council countries that although there is a "long list of things we fail to agree on", the humanitarian response should not be one of them.
She will say: "I've met Syrian refugees who have fled the bloodshed and violence consuming their country for over four years now.
"Their tales are of experiences that no one should have to go through.
"But, we must accept that these people have been let down. The generosity of countries bearing the brunt of the refugee crisis like Jordan, Lebanon and Turkey has not been matched by a similar generosity from the international community.
"UN appeals are 45% funded. Overwhelming suffering and loss has been matched by political deadlock and an inadequate financial response."
Ms Greening, who will become the first development minister to chair the UN body, will call on members of the Security Council to agree that all parties should follow international humanitarian law and allow humanitarian access to those in need.
She will say: "My message to the council today is that for Syria, for its people, for us all, time is now running out.
"Time is running out for us to meet the most basic needs of the Syrian people - whether they are inside Syria or have fled the country.
"Time is running out for Syria's children: a whole generation who are being robbed of a childhood, an education and a future.
"And time is running out for the international community as we try to cope with the overwhelming numbers of refugees who have run out of hope and are now looking elsewhere to build a new life for themselves and their families.
"As we meet today, there is a long list of things we will fail to agree on in relation to the Syria crisis.
"But help for those caught up in this crisis, humanitarian aid being able to reach those in need - these things shouldn't be on that list. These are things we should be able to agree to.
"There can be no excuse for flouting humanitarian law. There can be no excuse for preventing humanitarian agencies from reaching those in need. The Council must make that clear."