An explosive device was set off by unidentified individuals at the headquarters of the Mexican Bishops' Conference (CEM) in Mexico City early yesterday.

No one was injured in the incident, according to a conference spokesman, but a bishop said that it reflected the situation in Mexico, where three priests have been killed this year.

'The headquarters of the Mexican Bishops' Conference has been attacked with a three cylinder explosive device,' Bishop Ramon Castro of Cuernavaca said yesterday. 'I believe this reflects the situation in Mexico.'

The Catholic News Agency reported that the conference's press office director, Aramando Cavazos, said in a statement that 'today in the early hours of the morning, around 1:50 am, an unknown type of explosive device was placed at the main door of the CEM building'.

Cavazos added the explosion only caused 'material damage to that door' and said that 'no one outside or inside' was injured.

'The pertinent investigations are taking place, as apparently this is not the first case occurring in that area of Mexico City,' he said.

The incident follows the news earlier this month that a third priest had been killed in Mexico this year.

A statement from the local diocese said: 'With great pain and dismay we inform of the death of our diocesan priest Luis Lopez Villa, 71, who on July 5 was killed by criminals who managed to break into San Isidro Labrador Parish, in the municipality de Los Reyes.'

World Watch Monitor reports that Christians are often targeted because they stand against the drug trade by, for example, setting up rehabilitation centres. It says there are other reasons too. 'Christians are also targeted because of the perception that churches and their leaders have a lot of money, so congregations offer a ready source of cash – cartels can simply enter, lock the doors and ask the congregation to empty their pockets.'

A priest was killed in March, while a bishop went missing in January, only to be found dead later. These are just the latest incidents in a rising trend of violence towards clergy. Other Christians have also been targeted.

Mexico's drug trade has claimed numerous casualties in recent years. Christian Solidarity Worldwide said: 'The number of violations of freedom of religion or belief, specifically threats against and attacks on religious leaders by criminal networks, has risen in recent years across the country.'

In contrast to other North American countries, Mexico is predominantly Catholic, with a significant Protestant community and much smaller numbers of adherents of other religions.