Keep Hormuz open

President Reagan was appropriately emphatic at his news conference in insisting that there is ''no way'' the United States would permit Iran to carry out its threat to close the Strait of Hormuz. And he struck the right note in indicating that the US would seek to take any necessary action in concert with its allies.

Two points are at issue here. The most immediate one is the flow of oil: One-sixth of the world's petroleum reaches its markets by tankers that steam through the Strait of Hormuz, off the southern coast of Iran. Most of that oil goes to Japan and other nations of the Far East and lesser amounts to Western Europe and the US. If that petroleum were to be cut off for more than a very short time, the effect on the world's economies could be extremely serious.

Also important is the principle of free international access to the world's waterways. Several other key international waterways, from the Cape of Good Hope off South Africa to Malaysian straits, lie close enough to land that - at least in theory - nearby nations someday could threaten to close them. For the sake of world trade it is important that the principle be established now, in the Strait of Hormuz, that these lanes should be kept open.

Iran has repeatedly said that it would close the Strait of Hormuz if Iraq were to disrupt Iranian oil exports. With Iran this week apparently having launched another major offensive in its four-year war with Iraq, the threat is being taken very seriously.

Both the US and Britain have warships not far away in the Arabian Sea, should any action be deemed necessary.

It would be extremely difficult for Iran to physically close the strait as it has threatened. There are two shipping lanes in the strait, each a mile or more wide. Sinking enough ships to block them, or keeping them mined, would be very hard.

However, it would be less difficult to stop the tanker traffic through intimidation. If Iran were to attack one or two tankers - through air fire, mines, or shelling - insurance rates for tankers that used the area would quickly skyrocket. In that case few tankers would be willing to navigate the strait, thus effectively cutting off this supply of oil. It is against this contingency that other nations need to be particularly vigilant.