A line-up of US firms have taken roles on a landmark international dispute which has seen Ecuador ordered to pay $1.77bn (£1.1bn) plus interest in damages to oil and gas giant Occidental Petroleum.

The award – thought to be the largest ever granted under an investment treaty between nations – was made on Friday (5 October) by a tribunal of the International Centre for Settlement of Investment Disputes (ICSID).

The tribunal found that Ecuador improperly expropriated Occidental's oil drilling rights in 2006, when the country cancelled a 30-year oil concession. Ecuador's move was ruled to be in violation of the US-Ecuador bilateral investment treaty and international and Ecuadorean law.

Occidental was advised by Covington & Burling London partner Gaetan Verhoosel and associate Carmen Martinez Lopez, Debevoise & Plimpton New York partner David Rivkin and an Occidental in-house team led by Donald de Brier and Laura Abrahamson. Both Verhoosel and Martinez Lopez were at Debevoise when they first started work on the case.

Squire Sanders Cleveland partners George von Mehren and Stephen Anway acted for Ecuador, alongside Eduardo Silva Romero, Pierre Mayer and Jose-Manuel Garcia Represa in Dechert's Paris office, and Ecuador's attorney general Diego Garcia Carrion.

Ecuador is set to appeal the decision, with President Rafael Correa reported to have told the press: "We'll ask for it to be declared null and void. We're used to confronting these abuses, these obstacles, and we'll continue defending the integrity of the country."

The tribunal included ICSID president Yves Fortier QC, claimant-appointed David Williams QC and Brigitte Stern, who was appointed by ICSID on Ecuador's behalf.

ICSID, which is headquartered in Washington DC, is the international arbitration institution of the World Bank.