The World Health Organisation and UNICEF have said they are providing a team of experts to help investigate how 15 children died following a vaccination programme in northern Syria.
A joint statement said that they supported the suspension of the programme "for as long as the facts remain unclear" but that it was vital to start it again as soon as possible.
A spokesman for the Syrian opposition's interim government has said that a muscle relaxant was wrongly used in vaccinations against measles.
The relaxant, which had similar packaging, was used instead of the dilute for the vaccine in the second round of vaccinations.
"The investigation is continuing to find out who is responsible," Mohammed Saad said.
Fifteen children have died after receiving vaccinations against measles in the northern Syrian province of Idlib.
The children, who died on Tuesday, were under two years old and dozens more were made sick. It was initially feared that as many as 40 children died.
The vaccination programme, which began in Idlib and Deir ez Zor on Monday, has been suspended while an investigation takes place.
Video footage of British hostage Alan Henning filmed the day before his capture in Syria has been released in which he describes his trips to the country as "worthwhile".
The 47 year old was kidnapped while travelling with a humanitarian aid convoy shortly after crossing into Syria from Turkey.
"It's all worthwhile when you see what is needed, as you get where it needs to go. That makes it all worthwhile," Mr Henning says in the video, filmed on Christmas Day 2013.
"No sacrifice we do is nothing compared to what they are going through every day," he adds.
American fighter, attack and remotely piloted aircraft conducted seven airstrikes against Islamic State of Iraq and the Levant terrorists in Iraq yesterday and today, US Central Command officials have reported.
One airstrike destroyed an ISIL armed vehicle northwest of the Haditha Dam, two airstrikes destroyed two ISIL armed vehicles northwest of Irbil.
Four airstrikes southwest of Baghdad destroyed several small ISIL ground units and a small boat on the Euphrates River that was resupplying ISIL forces in the area, officials said.
All aircraft exited the strike areas safely, they added.
Centcom has conducted a total of 174 airstrikes across Iraq, officials said.
US President Barack Obama would consider requests to "forward deploy" US military advisers with Iraqi troops "on a case-by-case basis" if they are needed, a White House spokesman has said.
They would not be personally or directly engaging the enemy
Iranian president Hassan Rouhani has issued a strongly-worded condemnation of Islamic State terrorists he says are trying to "kill humanity".
In an interview with NBC News, Mr Rouhani said IS murders of civilians were contrary to Islamic teachings.
"From the viewpoint of the Islamic tenets and culture, killing an innocent people equals the killing of the whole humanity," Mr Rouhani said.
President Obama says "more than 40 countries" have offered to help in the fight against Islamic State terrorists.
He stressed that the US would not lead the fight alone, but would "do things differently" by building a strong international effort against the terror group, who are also known as Isis or Isil.
He told US military personnel: "We will lead a broad coalition of countries who have a stake in this fight because this is not simply America vs Isil, this is the people of the region fighting against Isil."
"It is the world rejecting the brutality of Isil and fighting for a better future for our children and our childrens' children - all of them," he added.
President Obama has made clear that the US will not send ground troops to fight Islamic State terrorists in Iraq.
Speaking at US Central Command in Florida, Mr Obama told military personnel: "As Commander-in-Chief I will not commit you and the rest of our forces to fighting another ground war in Iraq."
He said US personnel in Iraq were there to "support Iraqi forces on the ground as they fight for their own country against these terrorists".
A vaccination programme has been "temporarily suspended" in Syria after several children were reported to have died.Read the full story ›