NEW YORK - GAF Corp. has increased its stake in Union Carbide Corp. to 9.9 percent and said it will seek government approval to raise its holdings in the chemical company to as much as 15 percent.

On Wednesday Union Carbide announced a massive restructuring plan that involves layoffs of 4,000 employees and a $990 million charge against 1985 earnings in a move some analysts believe could be an attempt to ward off a takeover threat from GAF.

Moody's Investors Service said Friday it is reviewing Union Carbide's long-term debt for possible downgrading in light of the restructuring, which is expected to result in a loss for the company in 1985, and the potential for a takeover.

Union Carbide's actions appear to be in line with steps taken by other major industry players plagued by unprofitability in basic chemical operations and unrelated to legal claims filed against the company in connection with the disaster at its Bhopal , India, plant.

In a filing with the Securities and Exchange Commission, GAF said it now owns 6.96 million Union Carbide shares, or 9.9 percent of company. GAF said it purchased about 1.9 million of those shares between Aug. 26 and Aug. 28 at pricing ranging from $53.625 to $55.75 a share.

GAF said it will ask the Federal Trade Commission and the Justice Department for permission to up its stake in Union Carbide to 15 percent of the company's outstanding stock.

In an earlier filing with the SEC, GAF said it is buying Union Carbide stock for investment purposes.

Moody's said late Thursday that $1.7 billion of Union Carbide's long-term debt could be affected by the review. Moody's left UC's commercial paper, or short-term borrowings, Prime-2, one notch below the agency's top rating.

Although Moody's said Union Carbide's restructuring plans should improve long-term competitiveness, the rating agency said it must "weigh the consequences for creditors of other possibilities, including a takeover of the company," and the role other tactics in the restructuring could play in UC's "efficiency as a going concern." Among elements of the plan announced Wednesday are layoff of about 15 percent of its domestic staff, or 4,000 people, divestiture of "non-strategic" businesses, changes in depreciation policy, and a buyback of 10 million shares of stock.

In addition to uncertainty over the impact of these actions, Moody's also noted that "risk of material financial liability for Union Carbide continues to exist in relation to last December's accident in Bhopal , India, at the pesticide plant belonging to carbide's 50.9 percent-owned, independently operated subsidiary."