Beginning in the mid-1950s, the Soviet Union and China provided large-scale assistance. For example, China and the United States are involved with the expansion of the Sana'a International Airport. In the south, pre-independence economic activity was overwhelmingly concentrated in the port city of Aden. The seaborne transit trade, which the port relied upon, collapsed with the temporary closure of the Suez Canal and Britain's withdrawal from Aden in 1967.