A series of suicide bombings targeting Shiite religious processions in Lahore, Pakistan killed at least 25 people and injured up to 180 on Wednesday night, according to the city's administrative chief. It marks the first large-scale terror attack since flooding devastated Pakistan four weeks ago.

Though sectarian in nature, the attacks are likely to raise the level of threat felt by the hundreds of international aid workers who have come to Pakistan after the worst flooding in 80 years killed more than 1,600 people and left a fifth of the country under water at its peak.

“They will definitely be discouraged in continuing their activities in flooded areas, because those areas affected in the northwest were already vulnerable to militant activity," says Abdul Basit, a security analyst at the Islamabad-based Pakistani Institute for Peace Studies. "Foreigners become an attractive target to militants as [such an attack] provides them a wider platform to send their message to the world.”

Lahore was once considered a “safe” city in Pakistan, but has become a major target for militant violence in the past two years. In May, a set of coordinated attacks against the minority Ahmadi Muslim community killed some 93 people, while at least 50 Sufi Muslims were killed when a suicide bomber attacked the city’s Data Darbar shrine in July. In a separate incident Wednesday, at least seven were injured in the port city of Karachi when militants opened fire on Shiite worshipers participating in the Yaum-e-Ali ritual, which thousands of police were assigned to protect.

An affiliate of a Punjabi militant group, Lashkar-e-Jhangvi al-Alami, claimed responsibility for the Lahore attacks, according to Mr. Basit. Though the attack was not aimed at foreigners, a deteriorating law-and-order situation will be “discouraging” to aid workers.

Lashkar-e-Jhangvi is one of a number of militant organizations, which was previously devoted to harming Pakistan’s minorities but has in recent years formed linkages with the Pakistani Taliban and set its sights on the government.

On top of that, the Pakistani Taliban’s spokesman Azam Tariq last week warned foreign aid workers that their presence in the country is “unacceptable.”

The first blast took place when the main procession had ended and worshipers were sitting down to break the Ramadan fasts, according to Sajjad Bhutta, the district coordination officer of Lahore.

Though witnesses reported seeing police assigned to protect the procession fleeing the scene, Mr. Bhutta said a second suicide bomber detonated at a police checkpoint.

Following the blasts, angry mobs began pelting police and emergency rescue vehicles with stones, and set fire to a police station. Police responded with tear gas. Lahore’s top administrative official, Khusro Pervez, later admitted to local media that a security lapse may have occurred.

The procession in Lahore was held to mark the death of the first Shiite Imam Ali bin Abi Talib in the 7th century.