There are a "million reasons not to be hopeful" that a planned ceasefire in Syria will hold this weekend, a key US official has said.

Speaking for the first time to ITV News about his role, Barack Obama's senior Middle East advisor Rob Malley said that the US would still do everything it could to help end the five-year conflict.

The US and Russia reached a principle agreement last weekend to cease hostilities between various sides involved in the Syria conflict.

But the regime of Syrian President Bashar al-Assad - supported by Russian airstrikes - continued to bomb areas of the country held by what he called "terrorists", causing hundreds of civilian casualties.

"There's far more reason why it wouldn't work than it would work but we need to try, we need to do everything in our power," Mr Malley added.

"If others are not willing to do their part then it may well collapse, but we will not be caught having failed to do our best to make this work."

The so-called Islamic State, and other Islamic extremist groups, are also not taking part in the forthcoming ceasefire.

Any ceasefire, however shaky, can't come soon enough to the people of Syria.