Turkmenistan plans this week to sign a long-awaited agreement to supply natural gas to Pakistan and India through an ambitious US-backed pipeline that would cross Afghanistan, a source in the Central Asian country's government told Reuters Monday. Turkmenistan, which holds more than 4 percent of the world's natural gas reserves, plans to sign the sales and purchase agreement for the TAPI pipeline Wednesday, during an international gas conference in the Caspian Sea resort of Avaza. India's cabinet last week let state-run gas-firm GAIL Ltd. to sign a gas purchase agreement with Turkmenistan.