Despite the slip, the head of the mission, Sigrid Kaag, said she was still hopeful a June 30 deadline for the complete destruction of Syria's chemical arms would be met.
Syria's parliament meanwhile announced four new candidates have submitted applications to run in presidential elections scheduled for June 3.
And Iraq's interior ministry said Iraqi army helicopters attacked a jihadist convoy in eastern Syria as it tried to approach the border, killing at least eight people.
Speaking in Damascus, Kaag said 7.5-8.0 percent of Syria's declared chemical weapons material remained in-country, at "one particular site."
"However, 92.5 percent of chemical weapons material removed or destroyed is signficant progress," she said.
"We also however need to... ensure the remaining 7.5-8.0 percent of the chemical weapons material is also removed and destroyed."
Of that amount, 6.5 percent would be removed from Syria, she said.
"A small percentage is to be destroyed, regardless, in-country. That can be done. It's a matter of accessing the site," she added.
She acknowledged the security challenges facing the mission, but said Syria was required to meet its commitments nonetheless.
Under a US-Russian deal negotiated last year, Syria signed up to the Chemical Weapons Convention and agreed to hand over its entire chemical arsenal by June 30 of this year.
- Success 'around the corner' -
"Success is around the corner and this last push is very much needed," Kaag said.
"The 30 June deadline is around the corner... and we are hopeful that this is possible and will be met."
In addition to the remaining chemical material, there is a dispute over whether Syria will have to destroy 12 remaining chemical weapons production sites.
Damascus wants to seal the sites, which it says have already been rendered unusable, but Western countries want them completely destroyed, fearing that they may be reopened in the future.
There are also questions over alleged chlorine gas attacks in Syria in recent weeks, which the regime blames on a jihadist group but activists say were carried out by government forces.
Syria's government agreed to turn over its chemical arsenal last year as Washington threatened military action after a chemical weapons attack outside Damascus that reportedly killed some 1,400 people.
Activists and much of the international community blamed the attack on the regime, which denied responsibility.
New presidential candidates
Syria's parliament speaker on Sunday said four new candidates had announced they would compete in the country's June 3 presidential election, which is expected to return President Bashar al-Assad to power.
He has not yet announced his candidacy, though he has strongly suggested he will run.
The four candidates announced Sunday are Sawsan Haddad, Samir Maala, Mohammed Firas Rajjuh and Abdel-Salam Salameh.
Haddad, the only female candidate so far, was born in 1963 and is a mechanical engineer from Latakia province in the northwest, Assad's Alawite heartland.
Maala is an international law professor from Quneitra province in the south.
Rajjuh was born in Damascus in 1966 and Salameh, born in 1971, is from central Homs province.
They join a businessman, Hassan Abdullah al-Nuri, who studied in the United States, and independent MP and former communist Maher al-Hajjar as candidates.
The candidates are all largely unknown, with few details immediately available about their backgrounds or political leanings.
Elsewhere in the country, Iraqi helicopters opened fire on a convoy of tanker trucks in eastern Syria, Iraqi's interior ministry said.
"The army struck eight tanker trucks in Wadi Suwab inside Syrian territory as they were trying to enter Iraqi territory to provide the (jihadist) Islamic State of Iraq and the Levant (ISIL) with fuel," Brigadier General Saad Maan said.
He said the strike was not coordinated with Syria's government.
The vehicles were travelling towards the western Iraqi border province of Anbar, where ISIL has been battling Iraqi security forces and where militants have seized the town of Fallujah.