A rescue tube being driven through the rubble in a collapsed under-construction railway tunnel is believed to have hit one of the three trapped workers' vehicles and rescuers hoped they could reach the men on Tuesday.
A scanner and a sniffer dog had detected signs of human life on Monday evening.
The State Railway of Thailand (SRT) said that a rescue team from China arrived at the tunnel about 5am on Tuesday and would help.
The SRT also said it was possible the rescue tube had hit a large rock, not a vehicle. If so, they would dig around it before continuing the search.
Rescuers had detected the signs of the three trapped workers. Two were weak. They hoped to retrieve at least one of the men by 4pm on Tuesday.
The SRT was investigating the cause of the tunnel's collapse in Pak Chong district of Nakhon Ratchasima.
The collapsed tunnel is part of the Sino-Thai high-speed railway being built between Bangkok and Nong Khai province, which borders Laos.
A Chinese driver, a Chinese foreman and a Myanmar truck driver were trapped inside when a section of the planned 8-kilometre tunnel collapsed during construction on Saturday night. There were three vehicles in the tunnel.
