Serhiy Arbuzov's comment came after a day of talks with EU Commissioner Stefan Fuele in Brussels, which were being closely watched by anti-government protesters in Ukraine who are demanding such a deal.
"Ukraine will soon sign this association agreement with the European Union," Arbuzov said. That would mark a U-turn by President Viktor Yanukovych, who had refused to close such a deal with the European Union at a summit in Lithuania two weeks ago.
Fuele said such an agreement would lead to "bigger and bigger" EU financial aid to Ukraine, and that the bloc has "made a clear commitment to match in our financial support the level of ambitions of our Ukrainian partners."
Earlier Thursday, the bloc's foreign policy chief, Catherine Ashton, said that Yanukovych had told her he "intends to sign" at some point the trade and co-operation agreement he had rejected.
For weeks, activists have been amassed in Kyiv, Ukraine's capital, to protest Yanukovych's decision regarding the EU deal.
They are deeply concerned that he could instead sign an agreement to join a Russia-led customs union when he and Russian President Vladimir Putin meet next week.
Yanukovych appears to be in a tough corner. As protesters furious over his decision to turn away from the EU clog the centre of Kyiv, he appeared to be leaving his options open for the best deal he can get from his economically troubled country's powerful suitors.
Russia has put heavy pressure on Ukraine to join its bloc, which includes Belarus and Kazakhstan. Opponents say the bloc effectively tries to recreate the Soviet Union.
Yanukovych has said he is still open to the EU association agreement, if he can get a deal providing more aid to Ukraine, which is concerned about the impact of losing trade with Russia.
"We have explained the principle 'more for more,'" said Fuele, and his commitment on aid, including promises to top up some International Monetary Fund loans with more assistance could provide the breakthrough.
"Europe has yet again confirmed that the doors remain open to Ukraine, and the Ukrainian government yet again has confirmed that it still is on the same course to euro integration," Arbuzov said.
The Kyiv protests swelled to hundreds of thousands after police violently broke up two early rallies. Demonstrators are riding a wave of high morale after riot police stood down from two confrontations with protesters on Wednesday.
Protesters are reinforcing their positions in Kyiv's centre, erecting sizeable new barricades across streets leading to Independence Square.
Ashton, who talked with Yanukovych and opposition figures in Kyiv this week, said the short-term economic and financial issues Ukraine faces can be alleviated by signing the EU deal, which she said "will help to bring in the kind of investment that he needs."
"Look, Yanukovych made it clear to me that he intends to sign the association agreement," Ashton said on arrival for a meeting in Brussels early Thursday after her visit to Kyiv.
Ashton said Ukraine's economic problems "can be addressed by the support that not only comes from the EU institutions, but actually by showing that he has a serious economic plan in signing the association agreement."
Ukrainians in the east look more favourably on closer ties with Russia. Yanukovych, who is seeking a bailout loan from the International Monetary Fund to keep Ukraine from going bankrupt, is sensitive to the economic disruption that trade disputes with Russia can cause.
The EU association agreement sought to improve bilateral trade, streamline industry rules and bring about democratic reforms to promote justice and human rights. It stops short of offering EU membership but would open borders with Ukraine for increased commerce.
At the Vilnius summit where Yanukovych refused to sign the deal, two other eastern nations, Moldova and Georgia, initiated similar association agreements which will only need signatures from their institutions next year.
Jim Heinz reported from Kyiv, Casert from Brussels.
Follow Raf Casert on Twitter at http://www.twitter.com/rcasert and Jim Heintz at https://twitter.com/jeheintz.