US President Joe Biden has launched a "whole-of-government" response to oppose a new law in Texas that bans most abortions.
He called the Supreme Court's decision not to block the law an "unprecedented assault" on women's rights.
Any individual now has the right to sue anyone involved with providing or facilitating an abortion past six weeks of pregnancy in Texas.
This is before many women know they have conceived.
Rights groups had asked the Supreme Court to block the law, but it refused following a 5-4 vote.
The judges said their decision was not based on any conclusion about whether the Texas law was constitutional or not, and that the door remained open for legal challenges.
One of the liberal Supreme Court judges, Justice Sonia Sotomayor, said those justices who had allowed the law to stand had "opted to bury their heads in the sand" over a "flagrantly unconstitutional law".
President Biden accused the court of unleashing "unconstitutional chaos". "The highest court of our land will allow millions in Texas in need of critical reproductive care to suffer while courts sift through procedural complexities," he said.
Mr Biden said he had asked the Department of Health and Human Services and the Department of Justice to see what steps the national government could take to "insulate women and providers", but did not provide further details.
He said the law violated the landmark Roe v Wade case in 1973, in which the Supreme Court legalised abortion across the US. White House spokeswoman Jen Psaki told reporters that the president had long wanted to see the "codification" of Roe v Wade - which would mean Congress voting to make the precedent federal law.
Texas' Republican Governor Greg Abbott has said his state will "always defend the right to life".
The ruling was welcomed by Dan Patrick, Texas's Republican lieutenant governor, who tweeted: "A tremendous #prolife victory! This lifesaving legislation reflects Texas' pro-life beliefs and our continued commitment to protecting the most vulnerable."
How have abortion providers reacted?
Abortion providers in Texas were also looking for ways to challenge the new law, while dealing with its immediate impact.
"It has been truly devastating," said Andrea Ferrigno of Whole Woman's Health.
Patients have been "talking about their despair and how they've felt pressured and rushed to make a decision", she told the BBC.
For people already beyond the six-week deadline, she said Whole Women's Health had been offering information about what they could do next.
But she said the company was also "regrouping and consulting with our legal counsel" to decide on its next steps.
In the city of San Antonio, Planned Parenthood said it had paused its abortion services "while this plays in the courts".
Its director of public affairs, Mara Posada, said they were "getting calls and having to either let a person know that they cannot get an abortion after six weeks here in Texas, or if they are early enough in their pregnancy, we are referring them to a provider that is currently providing abortion care under the limits".
She added that in the week leading up to the law coming into force, they had seen twice the normal number of patients.
Hope Hanzlik, 21, had an abortion earlier this week, five weeks into her pregnancy. Her abortion provider told her they were having trouble finding appointments for everyone and that she was fortunate to get one.
How does the new law apply?
The so-called Texas Heartbeat Act prohibits abortions after what anti-abortion campaigners call a foetal heartbeat.
The American College of Obstetricians and Gynaecologists says the term "heartbeat" is misleading, and that what is being detected at this stage is "a portion of the foetal tissue that will become the heart as the embryo develops".
The Texas law enforces its ban with an uncommon approach: it empowers any private citizen to sue anyone who "aids and abets" an illegal abortion.
This means that an American may now be able to seek up to $10,000 (£7,200) in damages in a civil court against abortion providers and doctors - and possibly anyone involved in the process. People like clinic staff, family members, or clergy who encourage or support the procedure could, in theory, be sued.
Turning over enforcement of the Heartbeat Act to private citizens instead of government officials likely means that - in the absence of Supreme Court intervention - the law cannot be challenged until a private citizen seeks damages.
The legislation makes an exception in the case of medical emergency, which requires written proof from a doctor, but not for pregnancies resulting from rape or incest.
Texan women who wish to have an abortion after six weeks will need to travel to other states.
What is Roe v Wade?
Abortions were made legal across the US in a landmark 1973 Supreme Court judgement, often referred to as the Roe v Wade case.
By a vote of seven to two, the court justices ruled that individual state governments lacked the power to prohibit abortions.
The court's judgement was based on the decision that a woman's right to terminate her pregnancy came under the freedom of personal choice in family matters as protected by the 14th Amendment of the US Constitution.
The ruling came after a 25-year-old single woman, Norma McCorvey under the pseudonym "Jane Roe", challenged the criminal abortion laws in Texas that forbade most abortions.
Henry Wade was the Texas attorney general who defended the anti-abortion law.
The case created the "trimester" system that:
- gives American women an absolute right to an abortion in the first three months of pregnancy
- allows some government regulation in the second trimester of pregnancy
- declares that states may restrict or ban abortions in the last trimester as the foetus nears the point where it could live outside the womb