"So we're really in an unprecedented moment", de Blasio said.
A telephone bomb threat was received on February 20, and another one on January 31.
A Jewish community center in Whitefish Bay, Wis., was briefly closed Tuesday because of a threat over email.
Last week, St. Louis police arrested disgraced journalist Juan Thompson, 31, who was accused of making at least eight threats across the country - including an earlier one at the ADL's Manhattan headquarters - to get back at his former girlfriend.
Jewish groups nationwide have been coordinating with law enforcement over how to best respond to the threats, which have been made by a mix of people and robo-calls and some of which law enforcement officials say could be coming from overseas.
"First and foremost is safety and security, but we also have to send the message to the people trying to intimidate us that we're not going to be intimidated, we're going to go about our daily business", Trestan said.
India Hoists 360-ft Tallest Tricolour, Visible From Lahore in Pakistan
It is a proud moment for every citizen of India as the country's tallest tricolour was hoisted at Attari border on Sunday. Sources said that Pakistan has suspected that the Indian intelligence may use this pole for spying in Pakistan.
The threats - made via phone and email - are among more than 100 across the USA and Canada since early January, according to the Anti-Defamation League.
He spoke shortly after at least ten Jewish community centers across North America and four offices of the Anti-Defamation League received attack threats, in the sixth such wave since the beginning of the year.
The Anti-Defamation League said it was among the organizations across the country that were threatened Tuesday, only days after a string of similar calls.
Spicer said that as long as the threats continue, the administration will keep condemning them and look at ways to stop them.
New York Mayor Bill de Blasio met with Jewish leaders on Staten Island Tuesday then gave a news conference where he discussed the threats against Jewish organizations. The damage from these threats is far-reaching, often disrupting our classrooms and requiring the deployment of bomb squads and other SWAT equipment.
Last week, five grave stones were found toppled at the Waad Hakolel Jewish Cemetery, also named the Stone Road Cemetery, in the third such occurrence at a Jewish cemetery in a span of a few weeks. "We will follow up with more information as it becomes available".
The Chicago Jewish Day School consists of more than 200 students in pre-kindergarten through eighth grade.