"We believe that many of the West's policies on both Iran and Syria risk the stability and security of the Middle East," the Saudi ambassador to Britain, Prince Mohammed bin Nawaf bin Abdulaziz, wrote in a commentary in the New York Times.
"This is a dangerous gamble, about which we cannot remain silent, and will not stand idly by," he wrote.
The bluntly-worded warning was the latest in a series of public statements by senior Saudi figures expressing displeasure with US and Western diplomatic initiatives towards Syria and Iran.
Until recently, Saudi leaders rarely voiced public criticism of their Western allies in a decades-long partnership.
But Washington's decision to pull back from military action in Syria and its backing for an interim nuclear deal with Iran has dismayed the oil-rich Saudi kingdom, which views Tehran as a dangerous regional rival.
Citing Iran's backing for Syrian President Bashar al-Assad's regime, he said "rather than challenging the Syrian and Iranian governments, some of our Western partners have refused to take much-needed action against them.
"The West has allowed one regime to survive and the other to continue its program for uranium enrichment, with all the consequent dangers of weaponization," he wrote.
Diplomatic talks with Iran may "dilute" the West's will to confront both Damascus and Tehran, he said.
"What price is 'peace' though, when it is made with such regimes?"
As a result, Saudi Arabia "has no choice but to become more assertive in international affairs: more determined than ever to stand up for the genuine stability our region so desperately needs."
The Gulf monarchy had "global responsibilities," both political and economic, and he said: "We will act to fulfill these responsibilities, with or without the support of our Western partners."
In a thinly veiled jab at US President Barack Obama, the Saudi ambassador said that "for all their talk of 'red lines,' when it counted, our partners have seemed all too ready to concede our safety and risk our region's stability."
Obama had used the term "red lines" to warn Syria's regime against using chemical weapons. After the regime was accused of firing chemical weapons, Obama threatened punitive military strikes. But in the end he pursued a diplomatic agreement in which Damascus promised to give up its lethal arsenal of chemical agents.
The Saudi ambassador slammed the West for its reluctance to offer decisive help to Syrian rebels, vowing to continue support for the Free Syrian Army and the "Syrian opposition."
Acknowledging the threat of Al-Qaeda-linked groups in Syria, he argued the best way to counter the rise of extremists among the rebels was to support the "champions of moderation."