A man slashed a security guard in the neck inside a bank on the Upper East Side Friday morning, leaving the guard with serious injuries, police said.

The man tried to enter a Chase Bank on East 86th Street, at the corner of Second Avenue, shortly before 9 a.m., but a guard told him it was still closed, the NYPD and police sources said. 

Frustrated that the bank was letting its own employees in before 9 a.m., the man approached a teller after the bank opened and began acting “belligerent,” the sources said.

When a manager and a security guard told the man to leave, an argument ensued, and the man slashed the guard in the neck, according to the NYPD and the sources. 

The 59-year-old guard was taken to NewYork-Presbyterian/Weill Cornell Medical Center, where he was listed in serious but stable condition, police said. 

The NYPD initially said the guard had been stabbed and was unlikely to survive. 

The guard’s attacker — a man in a white shirt who appeared to be in his 20s — fled west on East 86th Street and had not been arrested as of Friday afternoon, police said. It wasn’t immediately clear what he slashed the guard with.

In a statement provided to NY1, Chase Bank said it was in touch with the NYPD about the incident.

“Our thoughts are with the security guard and his family and we are cooperating with police,” the bank said.

City Councilwoman Julie Menin, whose district includes parts of the Upper East Side, tweeted that she was “praying for the victim, and that the NYPD brings the perpetrator of this terrible attack to justice.”