Shanghai Pudong International Airport has beefed up security following two minor explosions on Sunday afternoon at about 2:26 pm.

"For the safety of passengers and aircraft, the Pudong International Airport has strengthened its security check measures. Passengers are asked to arrive at the airport earlier and to cooperate with the security checks," the Shanghai Airport Authority said on Monday.

The announcement was made one day after two bombs made from beer bottles filled with gunpowder exploded at a check-in counter at the airport's No 2 terminal, injuring four people.

The Shanghai Municipal Public Security Bureau said the suspect, Zhou Xingbai, a 29-year-old man from Gui-zhou province, had been mired in financial problems and after indulging himself in online gambling had "borrowed a lot of money".

"I'm prepared to do something really crazy that will definitely lead to my death," he said on WeChat.

The case is still under investigation.

"We advise our customers to arrive at the Pudong airport between three and four hours in advance," said Su Wei, an international air ticket broker.

Before the explosion, there was no security check at the airport entrance, and the zone where passengers undergo a security check with their carry-on baggage was located beyond the check-in area, according to Su Weiwei, an official of the Shanghai Airport Authority.

A passenger surnamed Zhang said that when he arrived at the airport he found a long line of domestic passengers going through security checks.

"The security check was going slower than usual and it took me 70 minutes to complete the procedure.

Passengers have to go through the procedure one by one, compared to four or five people at a time before the explosion," Zhang said.

A microblogger called Shanghai Agnes wrote that her aircraft taxied for nearly 45 minutes and passengers waited for more than two hours to board a shuttle bus after she arrived in Shanghai from Seoul.