WASHINGTON, June 14 (Reuters) - The government has assumed control of the first of four pension plans it plans to take over from bankrupt United Airlines , federal pension insurers said on Tuesday.

The Pension Benefit Guaranty Corporation took over the account representing 36,000 ground workers at the No. 2 airline on May 23, a spokesman for the agency said.

At termination, the plan had $1.3 billion in assets and $4 billion in promised benefits for total underfunding of $2.7 billion. The government says it will insure about $2.1 billion of the shortfall.

A bankruptcy court judge in Chicago approved a unique settlement last month between the pension agency and the airline that would result in the largest pension default in U.S. history, if completed.

US Airways has also terminated its employee pensions during its two turns through bankruptcy to help it restructure. The company is planning to merge with America West Airlines .

Under the deal to settle federal pension claims in bankruptcy court, United would terminate its four huge defined benefit plans covering 120,000 current and former workers in exchange for $1.5 billion in securities the deficit-ridden PBGC would receive to help meet its obligations.

The four plans covering pilots, flight attendants, ground workers and other employees were underfunded by nearly $10 billion and the government will guarantee $6.6 billion in benefits.

United hopes to save more than $640 million annually by shedding its traditional pensions and negotiating new, less expensive deals. The company says it has to drastically cut costs to attract investors.

Last week, a federal judge in Washington denied a bid by United flight attendants to block the PBGC from taking over its plan.