Police have released CCTV images of an "obese" man suspected of threatening children at a Sydney Islamic college with a knife.

The still images show the man on the grounds of the Al-Faisal College in Minto, in Sydney's south-west, wearing khaki pants and a green shirt.

Police have described him as being a Pacific Islander aged about 20 to 25 years.

Assistant New South Wales Police Commissioner Michael Fuller said there were very few people at the school on Thursday when the man entered the grounds.

"The person has walked in and made some comments, and asked some questions pertaining to the school," Assistant Commissioner Fuller said.

The man allegedly took a knife onto the college grounds and asked, "is this a Muslim school?" ( ABC News )

"(He) asked whether it was a Muslim school, and there is good CCTV footage of this Pacific Islander male, so we will be seeking the assistance of the community to identify him," he said.

Police say the man was armed with a knife when he entered the school on Benham Road, and that he spoke with a female staff member at the location, left a short time later and was last seen in Kitson Road.

Al-Faisal College, established in 1998, is an independent co-educational school with students from kindergarten to Year 12.

No-one was injured during the incident and security was tightened at the school, with police patrolling the perimeter as parents dropped their children off this morning.

Armed intruder 'verbally abusive': Islamophobia Register

Islamophobia Register Australia spokeswoman Mariam Veiszadeh said parents had told her the man was verbally abusive and sent the school into an emergency lockdown.

She said the man had asked a staff member if the college was a Muslim school.

"It was only at that stage that he continued to make threats," Ms Veiszadeh said.

"He was verbally abusive, yelling and saying all sorts of things," she said.

Farah Hannah, who has four children at Al-Faisal College, said she felt "very scared, like it's not safe here". ( ABC News: Mohamed Taha )

Ms Veiszadeh said a student who witnessed the incident ran into a prayer hall "absolutely hysterical and hyperventilating" to alert staff and students to the intruder.

"I also understand that at some stage there was an announcement made over the PA system to instruct everybody to ensure the school went into lockdown," she said.

Farah Hannah, who has four children at the school, said she did not feel safe.

"[I feel] very scared, like it's not safe here anymore," she said.

Earlier this week, two teenagers aged 18 and 14 were charged with yelling threatening comments at a Sydney Christian school while holding an Islamic flag.

Police allege the pair shouted anti-Christian comments from a car outside the Maronite College of the Holy Family in Harris Park last week.