Turkey has started hijacking net addresses as it steps up attempts to block access to social media.
Addresses belonging to Google, Level 3 and OpenDNS have all been hijacked by order of the Turkish government.
The hijack means that people using those addresses to reach Twitter or YouTube can no longer get through.
Net monitoring firms said the hijack was "concerning" and would let the government log who was trying to get round its controls.
The addresses that have been hijacked are for domain name servers - computers which list where websites are on the net.
One of the first ways Turkey blocked access to Twitter and YouTube was by getting ISPs to stop their domain name servers directing people to the two sites. It took action against the microblogging site and video service after both were used to leak information embarrassing to the government.
The government said it also imposed the blocks because the sites were spreading misinformation in the run-up to local elections which took place over the weekend.
To get around the blocks many people in Turkey told their computer to use domain name servers run by Google, Level 3, OpenDNS and many others. The alternative net addresses were shared by protesters in Turkey who painted the numbers on walls and buildings.
Now, the Turkish government has attempted to cut off this route by telling Turkey's ISPs to hijack the addresses and redirect traffic for them to a page saying the site cannot be found.
Google said it had received "several credible" reports that its domain name servers were being intercepted.
The latest action was "concerning" said Andree Toonk, an internet engineer at BGPMon in a blogpost, adding that it only usually saw this type of censorship in countries such as China. Via the hijack, Turkish ISPs could intercept traffic and censor websites "at will", he said, adding that the government could look at the traffic to see who is keen to use social media.
"It also has easy access to all queries being sent to these servers which allows for easy logging and recording without users noticing," he said.
The latest action by Turkey does not close all routes to Twitter and YouTube. Those who use virtual networks to reach the services will still get through and other domain name services will also work.
It is not clear how much effect Turkey's action has had on use of social media by protesters. The numbers of tweets sent in the country has grown and anonymous browsing service Tor reports that the number of people using its software has soared.
A Turkish court has also ordered the ban on Twitter to be lifted but it is not yet clear when that ruling will come into effect.