Spanish Trade Secretary Jaime Garcia Legaz said the EU would intervene over Argentina's seizure of the controlling stake in YPF from Spanish firm Repsol.
Foreign Minister Jose Manuel Garcia-Margallo said US Secretary of State Hillary Clinton also offered support.
But Argentine officials said they were not worried by possible reprisals over the nationalisation.
Argentine President Cristina Fernandez de Kirchner announced the measure on Monday, saying she was asserting sovereignty over Argentina's energy resources.
Her government is taking 51% of YPF, wiping out Repsol's 57.4% majority stake.
The move has wide support in Argentina but has provoked outrage in Spain, which has threatened reprisals.
The Spanish government will consider what measures to take at a cabinet meeting on Friday, and has also been rallying international support.
"There are going to be very clear interventions on the part of the European Union," Trade Secretary Jaime Garcia Legaz said, without revealing further details.
It is not clear what any reprisals might involve.
EU foreign ministers are expected to discuss the issue at a meeting in Luxembourg on Monday.
Spanish Foreign Minister Jose Manuel Garcia-Margallo said Spain would also raise the issue with the IMF, the World Bank and the G-20.
"We will try to pressurise Argentina so that it sees that this road is going nowhere,"  he said after meeting Hillary Clinton.
He added that Spain and the US had agreed to work together to "re-establish international legality".
But the Argentine government says it will not back down in its decision to nationalise YPF, which it says is a lawful action taken in the national interest.
"The government takes its decisions thinking about the Argentine people and not what the US or the Spanish government think," Interior Minister Florencio Randazzo said.
"We are not worried about any kind of reprisals," he added.
Repsol has said it wants around $10bn for its stake in YPF, but Argentina has said it does not accept that valuation.
It says YPF did not invest enough to increase output from its oil fields, forcing Argentina to rely on imports.
YPF, Argentina's biggest oil company, was privatised in 1993.
Last year it announced huge new finds of shale oil and gas.

The award was created in 2011 by BBC Radio 1 presenter Huw Stephens and promoter John Rostron.
This year's shortlist includes a mix of established artists and emerging stars, with songs that embrace a range of genres, sounds and influences.
Past winners of the prize include Gwenno and Gruff Rhys.
The 2016 shortlist of artists and their albums:
9Bach - Anian
Alun Gaffey - Alun Gaffey
Cate Le Bon - Crab Day
Climbing Trees - Borders
Datblygu - Porwr Trallod
Meilyr Jones - 2013
Plu - Tir a Golau
Right Hand Left Hand - Right Hand Left Hand
Simon Love - It Seemed Like a Good Idea at the Time
Skindred - Volume
Swnami - Swnami
The Anchoress - Confessions of a Romance Novelist
The winner will be chosen by a panel of industry experts including musician Amy Wadge, Dwynwen Morgan from BBC Radio Cymru and critic Lowri Cooke.
The winner will be announced on 24 November at The Depot in Cardiff.
Mr Rostron, from the Welsh Music Prize, said: "So many great albums are being released from Wales at the moment - it's a really good time to be listening to new music and finding some new favourites."
Spain says it is rallying international support against Argentina's nationalisation of the oil firm, YPF.
[NEXT_CONCEPT]
A shortlist of 12 artists has been announced for the Welsh Music Prize 2016.