Mass meetings will be held at Ford's Bridgend engine plant later amid claims by the Unite union that 1,160 jobs will go by 2021.
A decision to cut investment in a new Dragon engine project was announced in September.
Officials for the First Minister said the development was "clearly deeply concerning"
"We are actively seeking urgent assurances from Ford - both in the UK and US," said a spokesperson.
"We are also speaking to the workforce through the trades unions and offering them every assistance.
"We have worked very closely with Ford over a number of years to secure a future for the Bridgend plant, and we remain committed to supporting these vital manufacturing jobs."
Carwyn Jones is currently in the United States on a trade mission in Washington DC, where a £20m plane engine maintenance contract with GE Aviation for Wales has been unveiled.
The Unite union has called on Ford bosses to produce a five-year plan for the Bridgend factory
It said meetings being held on Wednesday meant workers were "one step closer to strike action in defence of their jobs".
Len McCluskey from the union said: "Unite members are committed to Ford, the Welsh Assembly is committed to Ford, all we need now is for Ford to commit to its workforce.
"That means investment, new product lines and a clear plan for the plant's future."
The plant currently makes 250,000 engines a year for Jaguar Land Rover and also 500,000 of Ford's own Sigma engines but it is due to stop producing both in 2018.
Five months ago, Ford announced Dragon production would be cut to 125,000 engines a year and investment would reduce from £181m to £100m.
The Welsh Government promised £15m state aid on the condition 500 jobs would be secured.