The power vacuum has left Kabul in a state of flux
Francesc Vendrell's visit comes amid fears that Afghanistan is disintegrating as groups jockey for position in the power vacuum.
The UN Security Council has endorsed a plan that includes convening a meeting of the various Afghan political and ethnic factions, probably in Europe or the Middle East.
However, Taleban leader Mullah Mohammed Omar has already said he would prefer to die rather than co-operate with the UN.
And in Kabul itself, various groups from across Afghanistan are staking their rival claims to power.
The Northern Alliance of Tajiks, Uzbeks and Hazaras are in control of the whole of northern Afghanistan and have promised to co-operate with the UN plan.
Troops debate
But it is also issuing its own invitations to representatives of Afghan groups to come to Kabul.
The Northern Alliance's political leader Burhanuddin Rabbani, who is still recognised by the UN as president of Afghanistan, had been expected to arrive in Kabul on Wednesday, but his visit was postponed without explanation.
M Vendrell told the BBC that the UN aimed to start with a small presence in Kabul and would expand its operations as conditions improved.
Meanwhile, the United States envoy to Afghanistan, James Dobbins, is holding talks in Pakistan on the plans for a transitional government. He arrived from Rome, where he met the former king of Afghanistan, Zahir Shah.
The fluid situation is reflected in other parts of Afghanistan - around the eastern city of Jalalabad, men loyal to Yunus Khalis, a Mujahideen party leader who has been largely silent for years, are now staking a claim in areas where they once fought Soviet troops.
Mr Khalis is not allied to the Northern Alliance, and the re-emergence of such independent operators is a sign of the factionalism into which Afghanistan could so easily fall once more.
Bargaining position
America and its allies are considering whether they need troops on the ground to stabilise the situation - a delicate matter, since the Northern Alliance has publicly discouraged the idea.
But the situation could also represent an opportunity, since no party is overwhelmingly dominant - provided the UN and the outside powers can get their act together quickly enough.
Meanwhile, a defiant Mullah Omar exclusively told the BBC's Pashto-language service: "We would prefer death to the government of fascists."
And he said a plan for the "destruction of America" was going ahead.
"The current situation in Afghanistan is related to a bigger cause - that is the destruction of America...
"The plan is going ahead and God willing it is being implemented, but it is a huge task beyond the will and comprehension of human beings.
"If God's help is with us this will happen within a short period of time.
"Keep in mind this prediction."