Image copyright Getty Images Image caption Sea lions are a popular tourist attraction in San Francisco

Part of San Francisco bay has been shut to swimmers following two rare attacks by sea lions.

A man was bitten in the groin area on Friday, a day after another man suffered a serious arm injury.

San Francisco's Aquatic Park will be closed until Monday, the authorities said, suggesting that just one animal was responsible for the attacks.

Although unusual, the local Marine Mammal Center says there have been 10 such attacks over a three-year period.

San Francisco Maritime National Historical Park tweeted that the Aquatic Park would be shut following reports of an "aggressive marine mammal".

Skip Twitter post by @SFMaritimeNPS The Aquatic Park Cove has been closed to swimming due to reports of an aggressive marine mammal biting swimmers in the area. The closure is scheduled until Monday, December 18, 2017. — SF Maritime NHP (@SFMaritimeNPS) December 15, 2017 Report

The latest victim was taking an early morning dip when he was bitten on the upper thigh.

It bit him "close to the family jewels" said Bob Roper, 79, who is a member of the same swimming club as the injured man, the San Francisco Chronicle reported.

Another man, 56-year-old Christian Einfeldt, was bitten on the arm on Thursday while swimming in the afternoon.

He told Fox affiliate KTVU he yelled "No" at the animal as it approached but it did not work.

"His head slid down my arm. Fortunately he only got one tooth into my arm," said Mr Einfeldt.

It is not clear why the swimmers were attacked. Experts warn that sea lions can be erratic.

"It could just be a sea lion being territorial, or it could be an injured or sick mammal," park spokesman Lynn Cullivan told CBS News.

In May, a sea lion pulled a young girl into the water at Steveston Fisherman's Wharf in Richmond, western Canada. She was unhurt.