WASHINGTON (Reuters) - President Donald Trump and Republican leaders in Congress will soon confront a complex challenge for tax reform: how to limit U.S. corporate tax avoidance schemes that take advantage of low tax rates in foreign countries. Congressional and administration staff have begun to examine options to address profit-shifting schemes that include so-called transfer pricing, earnings stripping and tax inversions.  A decision on how to handle these in tax legislation could come before Congress leaves town for its one-week July 4 recess on June 29, officials and lobbyists said. Lawmakers say the current tax code incentivizes profit shifting overseas because of the high 35 percent U.S. corporate income tax rate and rules that allow companies to hold profits abroad tax free until returned to U.S. soil. Without effective measures against tax avoidance, experts and lobbyists said tax legislation could trigger a new exodus of income and assets abroad. Because Trump and Republicans in Congress also want to end U.S. taxes on foreign earnings, companies could eliminate their U.S. tax bills altogether without restrictions.   Tax reduction strategies have been employed for decades by companies including Microsoft Corp (MSFT.O), Apple Inc (AAPL.O) and Amazon.com Inc (AMZN.O). Independent analysts estimate the federal government misses out on more than $100 billion a year in corporate tax revenues as a result of tax reduction maneuvers. That is equal to one-third of the $300 billion in annual corporate tax revenues. Many schemes seek lower corporate tax bills through “transfer pricing” - using transactions between business units to shift income abroad. The shift often coincides with the transfer of intangible assets such as intellectual property to low-tax nations where companies can expect single-digit tax rates.   Last week, Senate Finance Committee Democrats asked Treasury Secretary Steven Mnuchin to leave in place regulations adopted under President Barack Obama to combat earnings stripping and tax inversions. Companies use earnings stripping to shift income abroad as tax-deductible interest payments to foreign affiliates. Inversions are international mergers in which U.S. companies move their headquarters to foreign countries with low taxes, if only on paper, to lower their U.S. tax bills. Companies have accumulated some $2.6 trillion in abroad, equivalent to more than three-quarters of the $3.3 trillion in annual government receipts expected this year. BORDER-ADJUSTMENT TAX But the most effective measures against corporate tax avoidance schemes, including House Speaker Paul Ryan’s controversial border-adjustment tax, or BAT, have proved unpopular, raising the possibility that tax legislation could simply cut the corporate tax rate to 15 percent to reduce the advantages offered by foreign tax havens. Aside from BAT, which taxes imports but not exports, tax reform discussions are also looking at a minimum tax on profits from tax havens, a tax on intangible income and other measures to discourage companies from shifting profits to low-tax countries where they do little actual business, according to aides and lobbyists. Lobbyists said none of the options have enjoyed consensus support in Congress. Meanwhile, the idea of a simple rate cut does not sit well with House Republican leaders.   “Even with a low rate, we’ll continue to see U.S. jobs and research and headquarters move overseas,” said House Ways and Means Committee Chairman Kevin Brady, a leading BAT proponent. Experts warn that the 15 percent rate sought by Trump is well above a 5 percent effective rate that some corporations pay in countries like Ireland, the Netherlands and Luxembourg. Brady and Ryan are expected to address the issue in coming weeks with Mnuchin, White House economic adviser Gary Cohn, Senate Republican leader Mitch McConnell and Senate Finance Committee Chairman Orrin Hatch. The six are trying to forge legislation that could be unveiled as early as September. Trump has pledged the biggest tax overhaul since Ronald Reagan. But Republican infighting over healthcare has delayed the timetable. 