Leonardo Ulloa has threatened to go on strike at Leicester City and accused Claudio Ranieri of betraying him after the Premier League champions rejected an improved offer from Sunderland for the Argentinian striker.

Ulloa, who handed in a transfer request this month, took to Twitter on Monday to express his outrage after Sunderland’s latest bid – believed to be rising to about £7m – was turned down. “I feel betrayed by Ranieri and let down by the club. I will not play again for them,” Ulloa tweeted.

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In another move that has upset his employers, Ulloa gave an unauthorised interview to Sky in which he claimed Ranieri had reneged on an agreement to allow him to leave if Leicester’s valuation was met. “The manager’s been telling me for the last two weeks that if an offer of around £4m to £5m was arriving to the club, he would help me to leave.

“My understanding at this moment is that these offers are around those figures, or even bigger, [and] are not being considered, so after what Ranieri has been telling me for weeks, today he tells me that he wants me to stay.

“I can’t understand the situation or why they don’t let me continue playing my football somewhere else. If I stay at the club it’s going to affect seriously my career and my future. They know I’m not going to be used. The best thing for both parties is that they sell me to another club and I can continue playing my football somewhere else.”

Ulloa’s comments have gone down badly at Leicester and could lead to disciplinary action. For the moment, however, Leicester are focusing their attention on the remainder of the transfer window and have no intention of allowing Ulloa, who is injured with a thigh problem, to join a Premier League rival. Sunderland, bottom of the Premier League, are only six points behind Ranieri’s struggling side.

Leicester want to keep Ulloa come what may, with Ranieri eager to hold on to a player whose aerial threat up front provides his only alternative to Islam Slimani. Whether that stance would soften if a club from overseas matched Leicester’s valuation for the player is unclear. Alavés, the La Liga club, had a £1.7m bid knocked back earlier in the window and Galatasaray have also shown interest.

Asked about Ulloa’s situation, before the tweet and interview with Sky, Ranieri said: “I know he is very, very anxious but we don’t change our mind. For us it’s important. We don’t want to sell him. Also I don’t want to because I have just Slimani as a target man and I need another target man. I want Leo here and I hope he can be calm and understand my position. Of course he’s not happy and I understand him. The club know my idea very well and also Leo knows very well my idea. He is a fantastic player, a fantastic man and I believe in him.”