The Ministry of Information Technology is also blocking at least 17 links on Youtube and other websites for showing "blasphemous material."
"YouTube, Yahoo, Amazon, Bing, MSN, Hotmail and Google will be monitored with relation to anti-Islamic contents," said Khurram Mehran, spokesman for the Pakistan Telecommunication Authority.
The companies that own the affected sites are Google Inc., Microsoft, Yahoo and Amazon.com Inc..
But another official also made it clear the government had no intention of blocking major websites as they were important sources of education.
The move to impose monitoring was undertaken three days after a court in the eastern city of Bahawalpur ordered the government to block YouTube and eight other sites in response to a petition arguing they were showing material "against the fundamental principles of Islam.
The next hearing of the case is fixed for Monday. It is second time in a month that Pakistan has imposed such restrictions on internet.
Last month, authorities acting on a court decision blocked social network Facebook, YouTube and others sites for almost two weeks amid anger over a page that encouraged users to post images of the Prophet Mohammad.
BLASPHEMY A SENSITIVE ISSUE
Any representation of the Prophet Mohammad is deemed un-Islamic and blasphemous by Muslims, who constitute the overwhelming majority in Pakistan.
Blasphemy is a very sensitive issue in Pakistan. Five people were killed in protests in 2006 over publication of cartoons deemed blasphemous by Muslims in Danish newspapers a year earlier.
However, Latif Khosa, adviser to the prime minister on information technology, said the government had already been monitoring websites for any material prejudicial to "security of Pakistan and Islamic injunctions."
Khosa said the government could not block major search engines and websites as they were major sources of information and education.
"The constitution of Pakistan ensures access to knowledge, information and education to all citizens of Pakistan. These are the basic rights of the people of Pakistan and Internet is a major source of it," Khosa told Reuters.
Courts cannot violate those rights nor can any law be put in place to do so, he said.
"Many students are calling us and saying that they could not complete their higher studies if any step is taken to block these search engines," Khosa said.