Syphilis in the city is on the rise, and health officials aren’t sure why.

Instances of the chancre-causing disease spiked 8 percent in the first half of 2014, from 585 to 628 cases.

The disease has shown a steady increase long-term as well with 1,167 reported cases in 2013, compared with 117 cases in 2000.

Chelsea is the most hard-hit area. The most recent statistics showed an infection of nearly six times the citywide average.

Officials are scratching their heads, since studies show condom use is remaining steady.

Deputy Health Commissioner for Disease Control Jay Varna conceded Monday that the data is based on self-reporting, a potential flaw.

He said anonymous sexual hook-ups through the Internet are a hurdle to prevention because partners often don’t stay in touch.

Officials said the “vast majority” of syphilis cases are among men “who have sex with men.”