WASHINGTON – It took the better part of two years, but Congress has finally approved the potential transfer to Taiwan and Mexico of six frigates being decommissioned by the US Navy.

The move means negotiators have permission to make a deal for transfer of the ships, four of which are still in service.

Signed into law Thursday by President Barack Obama, the Naval Vessel Transfer Act of 2013 authorizes the transfer of the frigates Curts and McClusky to Mexico on a grant basis.

The act also authorizes the sale of the frigates Taylor, Gary, Carr and Elrod to Taiwan.

All the ships are of the Oliver Hazard Perry class, now being phased out of the US Navy. All remaining frigates are to be decommissioned by the end of September.

Gone from earlier versions of the bill are proposed transfers of frigates to Thailand and Pakistan. Those ships were removed from consideration by concerns about the May military coup in Thailand, after which the US government canceled numerous military deals and engagements with the country, and a number of political conditions attached to ongoing military aid to Pakistan.

The 2012 version of the Naval Transfer Act, containing a number of ships for Turkey, also failed, largely due to concerns in Congress about Turkish political policies.

Transfers of former US Navy ships to foreign allies is generally seen as beneficial to all concerned. In nearly every case, the ships are refurbished in the US before leaving for their new home , and US firms benefit from continued support of the ships in foreign service. Allies benefit from commonality with US Navy systems.

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Frigates are especially popular ships for foreign transfer, being relatively cheap to operate yet effective in a variety of roles. More than two dozen former US Perry-class frigates serve with foreign navies, while a number were built especially for other countries.

All the countries in the current and previous versions of the bill already operate former US Navy warships.

Taiwan operates eight Perry-class frigates, all built in Taipei by the China Shipbuilding Corporation. The additional frigates would be their first ex-US Navy Perrys, although the Taiwanese Navy operates older former US Navy frigates.

Mexico also operates six former US frigates of older vintage. If approved, these would be the first Perry-class frigates in Mexican service.

Three of the four ships approved for sale to Taiwan remain in US Navy service. Elrod is to be decommissioned in January, Taylor in May and Gary in August. Carr was decommissioned in March 2013 and is in storage at Philadelphia.

McClusky is scheduled to be decommissioned Jan. 14, while Curts was decommissioned in February 2013.

If a deal can be arranged, the US Navy and the foreign partners often prefer a "hot transfer," where the US crew removes certain items but the ship is left essentially intact so the foreign crew can simply walk on board after formal papers are signed. Such a transfer saves all parties money and the work of storing or reactivating the ships.

The Perry-class frigates were conceived in the 1970s as low-cost escort ships armed with surface-to-air missiles to protect convoys and amphibious ships. Early in this century, the US Navy phased out the Standard SM-1 missile that armed the ships and largely did away with external missile launchers in favor of vertical launch systems. None of the US Navy's current frigates retains a Standard missile capability.

As expected, China vigorously opposed any deal with Taiwan.

"This seriously violates the three China-US joint communiques, particularly the US commitments specified in the August 17 Communique," Foreign Ministry spokesman Qin Gang said Friday at a daily press briefing in Beijing, as reported by the official Xinhua news agency. The August 17 Communique was a 1982 US-China agreement to gradually reduce arms sales to Taiwan.

"It brutally interferes with China's internal affairs, sabotages China's sovereignty and security interests, and runs counter to the trend of peaceful development of relations across the Taiwan Strait," Xinhua quoted Qin as saying. "China expresses strong dissatisfaction and firm opposition to this and has lodged solemn representations with the United States both in Beijing and Washington. We reserve the right to take further action."