A "major milestone" in the battle to eliminate polio globally has been reached, the US Centres for Disease Control (CDC) has said.
Its experts think a second of the three forms of poliovirus has been eliminated after mass vaccination campaigns.
Wild poliovirus type 3 has not been detected for more than two years. Type 2 was eradicated in 1999.
Experts said the world was "closer than ever" to defeating polio but the situation in Pakistan was worrying.
Polio is highly infectious and causes paralysis in up to one in 200 people. Some children die when the muscles that help them breathe stop working.
But there has been huge progress in eliminating the disease. Cases have fallen from 350,000 in 1988 to 416 in 2013.
The last case of type 3 poliovirus was detected in Pakistan in November 2012, according to the CDC report.
"We may have eradicated a second of three; that's a major milestone," said Dr Stephen Cochi, a senior adviser at the CDC's Centre for Global Health.
However, a formal process - involving the Polio Global Certification Commission - is required before type 3 can be officially declared eradicated. That will not take place for at least another year.
Type 1 remains endemic in three countries - Pakistan, Afghanistan and Nigeria.
Dr Cochi told the BBC: "It's the most prickly one. For reasons that are unclear, this is the most common cause of polio outbreaks and the most frequent cause of paralytic polio."
There has been progress in Nigeria, where cases have fallen to six so far this year from 53 in 2013.
"But our biggest problem is getting worse in Pakistan," said Dr Cochi.
Cases have leap from 59 last year to 236 and counting in 2014.
The Pakistani Taliban stopped polio vaccination programmes in some tribal areas of the country for about two years.
Since the summer there has been a mass exodus of people from the region after military operations by Pakistan's army.
Dr Cochi added: "The good news is now those children are accessible in refugee camps or other parts of the country so they are getting vaccinated.
"But the bad news is the poliovirus has spread all over the country and there have been cases from Karachi and Punjab province."
It means there is a serious risk of polio spreading to other countries. The virus travelled from Pakistan to Syria in 2013.
Prof Walt Orenstein, from the vaccine centre at Emory University in the US, told the BBC: "Type 3 appears to be gone - I think it is overwhelmingly likely that we are there, but it's too soon to say we're definitely there.
"So it's not a total victory, but it is very promising."
But he warned: "Pakistan is a major concern, about 85% of wild type 1 poliovirus this year has been in Pakistan, but in Nigeria there is real hope we can get rid of type 1 even by the end of this year."