Google is targeting 100,000 terms associated with online child sexual abuse in a move hailed by David Cameron, who will announce a series of measures to tackle the problem at a cyber-summit in Downing Street. The prime minister said that Google and Yahoo had "come a long way" after the internet firms announced a series of initiatives to try to block access to images of child sexual abuse.

Google's executive chairman, Eric Schmidt, has announced that a 200-strong team has cleaned up Google Search to target 100,000 terms that can be used to locate child sexual abuse images. The changes will soon apply to more than 150 languages. The company is also showing warnings at the top of its search results for 13,000 queries.

In a groundbreaking move, YouTube engineers have created new technology to identify videos made by paedophiles. Google has been working with Microsoft to identify child abuse images.

The announcement came the day before a Downing Street summit at which the prime minister will announce that British and US law enforcement agencies are to jointly target online child abuse by monitoring those who operate on the hidden internet. A transatlantic taskforce will identify ways of targeting criminals and paedophiles who use secret encrypted networks to distribute images of abuse.

Schmidt says that the changes introduced by Google show that his company is listening. In an article for the Daily Mail, which has been campaigning to clean up the internet, the Google executive chairman writes: "We've listened, and in the last three months put more than 200 people to work developing new, state-of-the-art technology to tackle the problem. We've fine-tuned Google search to prevent links to child sexual abuse material from appearing in our results."

A spokesperson for Yahoo said the company had a "zero-tolerance policy when it comes to child abuse content online. Our dedicated governance and safety teams remove all illegal content across Yahoo properties. In our search partnership with Microsoft, each search is checked against Ceop's [the Child Exploitation and Online Protection Centre] blacklist of search terms and no results leading to child abuse images, videos, or abuse-related file sharing sites are shown."

The prime minister praised Google and other internet companies. "Google and Microsoft have come a long way," he told the Daily Mail. "A recent deterrence campaign from Google led to a 20% drop off in people trying to find illegal content, so we know this sort of action will make a difference."

The new Anglo-American law enforcement initiative extends already close co-operation between the UK and US in monitoring internet communications. The FBI and other US agencies will be asked to co-operate with their UK counterparts, including the new National Crime Agency.

Joanna Shields, the chief executive of London technology investment agency Tech City, who previously worked for Google and Facebook, will lead an industry group of experts exploring ways of targeting illegal activity online.

Shields said: "It's vital that governments and industry work together to eradicate child abuse content from the internet, and that we mobilise the best and brightest in the technology industry to come up with innovative solutions."

The government estimates that 20,000 people are using encrypted and anonymous networks, such as the Tor anonymising service, to communicate. Many of these will be carrying out perfectly legal activities.

Awareness of the threat to privacy and knowledge of the "dark web" have increased since news of the US National Security Agency's activities surfaced. But the dark web is also a favoured tool for those involved in distributing child abuse material, who have an interest in keeping their identities hidden.

The dark web is so called because sites are hosted in such a way as to be inaccessible through the open internet, and cannot be found by standard search engines.

In order to access them, users must first download special software. When they access the sites, the technology used means that they do so with a high degree of anonymity. The location of the server used to host a particular site is also protected by layers of encryption.

Downing Street said: "Child abuse online is not restricted by international borders and so neither must our response be."

The meeting at No 10 will be attended by representatives of BSkyB, TalkTalk, BT, Virgin Media, Google, Microsoft, the National Crime Agency, the NSPCC and the Internet Watch Foundation.

It comes four months after Cameron promised to tackle online child abuse, saying it "pollutes the internet". He vowed to end the proliferation of child abuse images on the internet and ensure that children were protected from viewing damaging material.

• This article was amended on 18 November 2013 to correct Eric Schmidt's title from chief executive to executive chairman