More than 20 flights of Indian airlines received bomb threats on Sunday, according to sources.

IndiGo, Vistara, Air India and Akasa Air are among the airlines that received bomb threats, including international flights, they added. Six flights from IndiGo, Vistara, and Air India received threats.

In separate statements, an IndiGo spokesperson said the airline is cognisant of a situation involving flight 6E 58 (Jeddah to Mumbai), 6E87 (Kozhikode to Dammam), 6E11 (Delhi to Istanbul), 6E17 (Mumbai to Istanbul), 6E133 (Pune to Jodhpur) and 6E112 operating (Goa to Ahmedabad).

Vistara said it got security threats for six flights — UK25 (Delhi to Frankfurt), UK106 (Singapore to Mumbai), UK146 (Bali to Delhi), UK116 (Singapore to Delhi), UK110 (Singapore to Pune) and UK107 (Mumbai to Singapore).

“In line with the protocols, all relevant authorities were immediately notified, and security procedures, as directed by them, are being carried out,” a Vistara spokesperson said in a statement.

An Akasa Air spokesperson said some of its flights received security alerts on Sunday.

Emergency response teams are monitoring the situation and are in touch with the security and regulatory authorities, the spokesperson said in a statement.

“We are following all safety and security procedures in coordination with local authorities,” the spokesperson added.

While the sources said that there were threats for at least six Air India flights, there was no immediate comment from the airline.

So far this week, more than 90 flights have received bomb threats and most of them have turned out to be hoaxes.