Public transport workers in Northern Ireland are planning four further days of strike action in February as part of a dispute over pay.
In a joint statement, Unite, GMB and Siptu said the first walkout will take place on Thursday 1 February .
No further dates have yet been announced.
The unions said it marks a "further escalation in the industrial action of bus and rail workers demanding
Secretary of State for Northern Ireland Chris Heaton-Harris release funding for public transport services and allow a cost-of-living pay increase".
Public transport workers were among those who took part in mass industrial action in Northern Ireland last week.
Prior to that, train and bus drivers also participated in a number of days of strike action before Christmas, bringing Translink services to a halt.
Unite general secretary Sharon Graham said: "Last week's strike action in Northern Ireland represented an historic mobilisation of workers demanding Chris Heaton-Harris release funds to provide a pay increase for public service workers.
"In the absence of any movement on pay, public transport workers have been left with no alternative but to escalate their strike action with further strikes.
"They can be assured of the continued support of Unite in that fight."
GMB regional organiser Peter Macklin said: "Chris Heaton-Harris's behaviour to date to workers, our public services and indeed the public has been nothing short of disgraceful.
"His attempts to weaponise the funding of public services and pay have backfired, leaving him increasingly isolated.
"He is solely responsible for the current wave of strike action. He must release the funds which he is withholding from workers and public services."
Siptu regional organiser Niall McNally said: "Public transport workers demand the release of the funds needed for a cost-of-living pay increase.
"Workplace reps have unanimously agreed an escalated schedule of industrial action, including four further days of strikes in February.
"The three unions are also seeking to co-ordinate our action with that of the wider trade union movement to keep the maximum pressure on Chris Heaton-Harris so that we can win for workers and our public services."
During negotiations about the restoration of the Stormont powersharing institutions in December, Mr Heaton-Harris offered a £3.3 billion (€3.9 billion) package to stabilise finances in Northern Ireland, including £600 million (€700 million) to settle public sector pay claims.
However, the government has insisted the package is dependent on a deal to restore Stormont.
The Northern Ireland parties have said the Northern Ireland Secretary should divorce the issue of public sector pay from the wider financial offer and settle the workers' claims immediately.
Mr Heaton-Harris has, however, insisted he does not have the power to intervene in what is a devolved issue.
The Northern Ireland Office has been contacted for comment.
