Washington (CNN) US Defense Secretary James Mattis warned Saudi Arabia on Tuesday that support for the Saudi-led coalition in Yemen is "not unconditional" and urged them to "do everything humanly possible to avoid any innocent loss of life."

His remarks came after the Pentagon issued a warning to the Saudis that it is prepared to reduce military and intelligence support for its campaign against rebels in neighboring Yemen if the Saudis don't demonstrate they are attempting to limit civilian deaths in airstrikes following a strike on a school bus that killed 40 children earlier this month.

Two officials directly familiar with the Pentagon's thinking tell CNN frustration is rising. Mattis and General Joseph Votel, head of US military operations in the Middle East, are particularly concerned that the US is supporting a Saudi-led campaign of airstrikes that have killed large numbers of civilians.

Human rights groups, some members of Congress and the United Nations have expressed concern about Saudi Arabia's actions for months to little avail. But after a series of recent strikes in which large numbers of civilians were killed, the Pentagon, as well as the State Department, have now delivered direct messages to the Saudis about limiting civilian casualties. "At what point is enough enough?" one official remarked to CNN.

Speaking at the Pentagon, Mattis said the US is "constantly reviewing" the support given to the Saudi led coalition in Yemen.

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