A UN agency has described the eruption of polio in Syria as perhaps "the most challenging outbreak in the history of polio eradication" after the number of cases in the war-ravaged country reached 38 and the first case was confirmed in neighbouring Iraq.

According to the World Health organisation (WHO), the Iraqi case – found in a six-month-old unvaccinated child in Baghdad – is related to the outbreak in Syria, fuelling fears that the virus is spreading around the Middle East.

"The current polio outbreak in Syria – now with one confirmed case in Iraq – is arguably the most challenging outbreak in the history of polio eradication," said a spokesman for the UN relief and works agency for Palestine refugees (UNRWA).

"Seriously damaged health infrastructure, poor health access and utilisation because of insecurity inside Syria, and massive movements of vulnerable and at-risk populations in and out of Syria – all make controlling the outbreak and rendering health protection to Palestine refugees in Syria and across the region very challenging."

The same factors, he added, made it hard to guarantee 100% immunisation coverage and to maintain the cold chain needed to protect vaccines from heat.

The UNRWA is part of the team, led by the WHO and Unicef, that has fought to contain the virus since it was detected in Syria for the first time in 14 years last October. Until this week, Iraq had not reported a case since 2000.

In the five months since polio was confirmed, more than 22 million children in seven countries – Syria, Lebanon, Jordan, Turkey, Iraq, Egypt and Palestine – have been vaccinated as part of the largest vaccination campaign in the history of the Middle East.

"Since we got the confirmation of the outbreak at the end of October last year, the response was as rapid as possible," said Juliette Touma, a spokeswoman for Unicef.

"Have we reached every child we wanted to reach? The answer is no, we haven't, and this is largely because of access restrictions. The severity of the conflict makes people move all the time and we have displacement on a daily basis, so the ability to control the disease is a challenge."

The aim, said Touma, was to vaccinate children several times: six times in Syria and three in the wider region. But she added that there was no way to guarantee that the spread of the disease could be arrested.

"As long as we don't have unhindered access inside Syria to areas that are under siege and that are hard to reach, polio will not be contained," she said. "We always say with polio that there's no borders; there's no checkpoints. The virus doesn't need a passport – it just travels."

The WHO, which has faced allegations from some quarters that it was too slow to react to the outbreak, is adamant it has moved as quickly as possible.

Sona Bari, a spokeswoman for the organisation's Global Polio Eradication Initiative, said the WHO had detected the virus in sewage in Egypt and Israel more than a year ago and issued an international high-risk alert despite an absence of confirmed cases.

"In October [last year], before cases even were confirmed in Syria – as soon as there was the first cluster of suspected cases – we issued another alert," said Bari. "When the cases in Syria were confirmed, an international emergency was declared and these seven countries put together a co-ordinated response plan to cover about 22 million children."

She said the WHO had been heartened by the increasing numbers of children being vaccinated in Syria, adding that the most recent round of vaccinations had probably reached about 3 million children.

Bari brushed off the criticisms of a US paediatrician who had accused the WHO of acting too late and failing to effectively vaccinate children at risk.

"The thing to note is that neither the opposition groups nor the government is stirring these allegations about WHO," she said. "I think those allegations are motivated by a desire to do something for the children of Syria; I think the motivation is absolutely laudable. [But] I think targeting WHO doesn't really achieve anything; what we need to do is actually all concentrate together on vaccinating Syrian children."