Barca are in talks with Arsenal to buy the Spain midfielder, a long-term target for the Spanish and European champions, Guardiola, flanked by sporting director Andoni Zubizarreta, told a news conference.
"This year Arsenal has agreed to negotiate and we are working on it," Guardiola said.
"Barcelona has made an offer, Arsenal another and we have time until August 31 and we'll try to reach an agreement," added the 40-year-old.
"There is a sum of money in the strong box set aside for this signing but if it doesn't work out it will be kept in the box for something else.
"We will fight to the end to try to get Cesc because we believe he will improve the team and the squad."
Fabregas, 24, came through Barcelona's youth academy at the same time as World Player of the Year Lionel Messi before joining Arsenal in 2003 at the age of 16.
He discussed his situation with Arsenal manager Arsene Wenger when he returned for pre-season training this month and did not join his team-mates on a tour of Asia due to injury.
Wenger is trying to convince Fabregas there would be no greater achievement for him than to lead Arsenal to success and that it is not the right time for him to leave England.
Guardiola said that despite the push to land Fabregas, his priority was to sign a forward and Zubizarreta said the club were close to completing the purchase of Chile striker Alexis Sanchez from Serie A side Udinese.
Barca were also on the verge of selling striker Bojan Krkic to AS Roma, who have hired former Barca B coach Luis Enrique to lead them for the coming season, Zubizarreta added.
"As we understand it we are in the final stages of the negotiations [to sign Sanchez] and an agreement will happen very soon," former Barca and Spain goalkeeper Zubizarreta said.
"But I would have said the same thing this time last week. As in a good film, it seems sometimes the end takes a little longer," he added.
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