Manchester United's attempts to sign Robin van Persie continue to drag on with currently no end in sight. Arsenal have rejected United's latest offer of £15 million, according to the Daily Mail. Reportedly it is the great rivalry between the two clubs and their managers that is posing a major stumbling block.

With the new season now under a week away, the situation could be coming to head, however. Van Persie appeared as a second-half substitute in Arsenal's friendly with Cologne on Sunday and was promptly booed by his own club's fans.

Despite the reaction, manager Arsene Wenger claimed that he still hopes to hold onto the Dutchman.

"There's no news on Van Persie," he said, according to the Sun. "Our fans want him to stay so you can understand they are excited.

"Anything I could say about Van Persie would be a lie because he's our player and that's it.

"Robin scored 30 goals last season and you want to keep your best player.

"He was player of the year in England. We want to keep him. I cannot tell you more at the moment because I don't know more."

If United were to offer in excess of £20 million, whether Arsenal would risk losing the 29-year-old for nothing next summer remains to be seen.

If talks do not progress, United could look to alternatives to boost their attacking options.

Polish publication Onet.Sport claims that United have launched a €30 million (£23.6m) bid for Borussia Dortmund striker Robert Lewandowski.

United were linked to the Polish striker in the early part of the summer as they also wrapped-up a deal for his former teammate Shinji Kagawa. The 23-year-old had a breakthrough season last term as he struck 22 goals to lead Dortmund to their second-consecutive Bundesliga title. He then followed that up with the opening goal of the European Championships for the co-hosts.

Having already lost Kagawa, Dortmund will be extremely reluctant to lose yet another of the key players from their German double-winning side.

In contrast, there now appears to be a good chance of another reported United target, Fernando Llorente, leaving his club Athletic Bilbao.

With just a year remaining on his contract with the Basque club, Llorente has told his employers that he will not be extending his deal.

"He and his agent informed us that his decision was that he would not extend his contract," president Josu Urrutia said, according to Reuters. "Essentially there was no economic reason and what had the most weight was choosing to take part in a different type of competition with more chances of success."

Llorente continued his fine goalscoring form last season as he helped Athletic to the final of the Europa League. But with the club having fallen away in the league at the climax of the campaign, the 27-year-old is now eager to compete for the game's biggest honors.

Juventus are thought to be in pole position to sign the towering front man, with United as well as local rivals Manchester City also said to be interested.